The Pali Canon and Mahayana texts agree that the Buddha allowed his monks to eat certain kinds of meat.
In the Pali Canon, the Buddha said:
"I say that there are three instances in which meat may be eaten: when it is not seen, not heard, and not suspected, that the living being has been slaughtered for the bhikkhu."
Such allowance of meat eating is acknowledged in the Mahayana texts but these texts also declare that the permission is being revoked and henceforth no meat eating is to be allowed whatsoever. In the Pali Canon there is no mention of Buddha revoking this allowance.
So why was meat eating allowed? In the Pali Canon it is clear that meat eating is not regarded as defiled behavior. In the Amagandha Sutta, a vegetarian Brahmin confronts Buddha Kassapa in regard to the evil of eating meat to which Buddha Kassapa replies as follows:
"Taking life, beating, cutting, binding... adultery; this is uncleanliness and not the eating of meat.
When men are rough and harsh, backbiting, treacherous, without compassion.... this is uncleanliness and not the eating of meat.
Anger, pride, obstinacy, antagonism.... this is uncleanliness and not the eating of meat.
When men are of bad morals, refuse to pay their debts.... this is uncleanliness and not the eating of meat.
When men attack living beings either because of greed or hostility and are always bent upon evil, they go to darkness after death and fall headlong into hell; this is uncleanliness and not the eating of meat."
So as far as the Pali Canon goes, the Buddha sees no direct connection between defiled states of the mind and meat eating which is, I believe, the main reason he allowed his monks to eat meat (with a few exceptions).
However, when we look at the Mahayana texts to determine why meat eating was allowed, the answers seem unsatisfactory.
In the Nirvana Sutra:
Kasyapa said: "Why did you first allow the bhiksus to eat three kinds of pure meat?" "O Kasyapa! These three kinds of pure meat were so instituted following the need of the occasion."
Here the Buddha is saying that there was some kind of necessity for the allowance. Unfortunately there is no elaboration of what this necessity was. One possibility is that in the mountainous areas there was a scarcity of fruits and vegetables. But if that was the case then surely the rule would have been formulated differently and be limited to monks living in areas where fruits and vegetables were not readily available.
Since the Buddha is also revoking the allowance there is thus the question of how the situation has suddenly changed so as to make the allowance no longer necessary. This question is also left unanswered by the sutra.
Now when it comes to the Lankavatara Sutra, this is said:
Some people will say: 'In other sutras, the Tathagata has allowed eating of three kinds of meat', they simply do not understand that the meaning of precepts is to stop meat eating gradually, so they will say that meat eating is allowed.
Here, the reason is no longer about necessity, but rather to "stop meat eating gradually". This reason also seems unsatisfactory. First, limiting meat allowed to the three types would probably not prevent the monks from eating meat most of the time. It may have been more effective to restrict the times when meat eating would be allowed, eg. allowing pure meat to be consumed only on the first day of each week or something like that. Secondly, given that meat eating is considered as very evil behavior that could send one to hell, it doesn't make sense why the Buddha would opt for gradual elimination rather than an immediate, total ban. Were monks really so addicted to meat eating that it required gradual elimination? What about the ban on sexual intercourse? What's more difficult for a man, a celibate life or a vegetarian diet? I would say that in most cases it is a celibate life, but the Buddha set a total ban on sexual intercourse. Why not go for gradual elimination and specify certain types of pure women whom the monks would be allowed to copulate with?
So the reason for the allowance given in this sutra also seems unsatisfactory.
As for the Surangama Sutra, this is stated:
"I permit the bhiksus to eat only the five kinds of pure meat. Such meat is actually the product of my transcendental power of transformation and not of animal slaughter. You, Brahman, live in a country where vegetables do not grow because it is too damp and hot and because of all the gravel and rock. I use my spiritual power of compassion to provide you with illusory meat to satisfy your appetite. How then, after my nirvana, how can you eat the flesh of living beings and so pretend to be my disciple?"
So there is yet another different reason given for the allowance of pure meat. Here, the Buddha is saying that the meat the monks have been eating are actually magic meat created by him ie. fake meat that didn't really originate from a slaughtered animal. He says he does this out of compassion to satisfy the appetites of monks. This doesn't seem to be the way the Buddha usually teaches. While he does have limitless compassion, nonetheless, it is not usual for him to express this compassion by satisfying the sensual desires of his monks.
Another curious element is the assertion that India was a country where "vegetables do not grow". Is this an accurate description of India at that time? I think it should only apply to the mountainous areas of India. Even if there were no vegetables, then why didn't Buddha create magic vegetables for his monks to eat. By creating magic meat, wouldn't the Buddha cause the monks to think they were eating real meat and wouldn't that produce defiled states of mind in the monks as described in the Mahayana texts?
Again, the reason for the allowance given in this sutra is unsatisfactory.
In fact, when we look at the Mahayana texts' treatment on the issue of meat eating and use of animal products we can find a number of other inconsistencies and even conflicts. For example, let's take the issue of animal products.
In the Surangama Sutra, it is said:
"Bhikshus who do not wear silk, leather boots, furs, or down from this country or consume milk, cream, or butter can truly transcend this world... Both physically and mentally one must avoid the bodies and by-products of living beings, by neither wearing them nor eating them. What I have said here is Buddha’s teaching. Any explanation counter to it is the teaching of Papiyan (demon king)"
However, in the Nirvana Sutra the following is stated:
Kasyapa said: "If the Tathagata means to prohibit the eating of meat, such things as milk, cream, fresh butter, clarified butter, and sarpirmanda, all kinds of clothing, silk cloth, horse-shoe shell, hide and leather... should not be received."
[The Buddha replied:] "O good man! Do not muddle things up with what the Nirgranthas [Jains] say. Each of the prohibitions which the Tathagata lays down has a different meaning."
So in this case, we have a direct conflict between two sutras. One says that animal products such as milk, leather and silk must be avoided while another says that believing so would be muddling up the teachings of the Buddha with that of Jainism. So which is correct?
We can take another example which is whether meat eating can be considered as equivalent to killing.
In the Lankavatara Sutra, it is said that:
"Mahamati, if no one eats meat, then no one kills living beings for food. Because there are people who wish to eat meat, if they have no meat to eat they will go everywhere to buy meat, then the others who wish to earn money will kill living beings and sell the flesh to the meat eaters. The killings are for the buyers, thus the buying is the same as the killing."
"Hunters, butchers, meat eaters and the like are atrocious and hardhearted, they can do what others cannot have the heart to do. When they see the living beings whose bodies are fresh, fat or fleshy, they would think of eating them."
However, in the Brahma Net Sutra, meat eating is listed as one of the 48 secondary precepts while killing is listed as one of the 10 major precepts. The major precept of killing is described as follows:
"A disciple of the Buddha shall not himself kill, encourage others to kill, kill by expedient means, praise killing, rejoice at witnessing killing, or kill through incantation or deviant mantras. He must not create the causes, conditions, methods, or karma of killing, and shall not intentionally kill any living creature."
Therefore, since meat eating entails just a breach of a secondary precept, it follows that meat eating does not amount to killing otherwise, meat eating would have to be listed as a major precept rather than just a secondary precept. Therefore, this is in conflict with the Lankavatara Sutra which sees meat eating as equivalent to killing.
So what does all of this imply? We see that there are inconsistencies and sometimes direct conflict among the various Mahayana texts dealing with the issue of meat eating and the use of animal products. Explanations given in the Mahayana texts about why monks were permitted to eat certain types of meat also seem to be unsatisfactory. So if one were to rely on the Mahayana texts to determine what were actually the Buddha's words on these matters, the existence of inconsistencies and conflicts could cause some confusion. And in answering the question of "Why did the Buddha allow his monks to eat meat" we could say that this question can be more satisfactorily answered by reference to the Pali Canon.