Are all Buddhists vegetarian? Certainly not. Some are quite strong on insisting that vegeterianism is not a Buddhist practice, others believe a "true" Buddhist should also be a vegetarian.
Here are some views I have collected from monks of different Buddhist schools over the years. I shall simply present them without trying to decide which one is "right". After all, there is no dogma in Buddhism. I have always maintained that deep down there are as many Buddhisms as there are Buddhists - and I like it that way. At the end, however, I will present my own practice and my reasons for it.
All Theravada monks I either talked to, or read books by, were non-vegetarian. They reminded me that the Buddha himself ate meat, and indeed died after eating a piece of spoiled pork. The Buddha said, "Eat what is put into your bowl." In Thereavada countries monks still practice walking down the street with a begging bowl, which Buddhists (non-monks) fill with whatever food they have left. That is what the monks eat, be it meat or not.
Secondly, when presented with the argument against killing, the Theravada monks said that vegetables are killed as well, and they do not wish to view animals as superior beings as opposed to vegetables.
In Mahayana countries, due to different geographic and political conditions, the practice of walking down the street with a begging bowl is not common. Some Mahayana monks are vegetarians, others are not.
According to a Zen teacher trained in the Chinese tradition (which would technically make him a Ch'an teacher), vegetarianism in Buddhism developed in China because monks were often invited to eat in people's homes. That was before refrigeration, so people would say, oh there you are, let me go and kill a hen for you. To that, the monks would say, absolutely positively not! The first precept of Buddhism is "do not kill."
The Chinese monks, according to this teacher, were willing to eat meat if the animal was already dead, but they would not allow an animal to be killed for them. They also made that very clear to Chinese people, so people would eventually stop even offering meat to monks. Over centuries the original reasons apparently became unknown to most, and people (both monks and others) just believed that a Buddhist should be a vegetarian. It even went so far that many Chinese believe the Buddha was a vegetarian and died from eating poisoned mushrooms.
My Practice
As promised, I will now present my personal practice, and my reasons for it.
First off, I never cared for meat, even as a child, long before I heard of Buddhism. I would often take a bite, then chew it and chew it until my mother would say, OK, go spit it out. So, in a way, I am a natural vegetarian, perhaps for reasons that go back to a previous life.
As a young adult, I did eat meat, and especially loved steak. Though, occasionally, I decided to be a vegetarian, and practiced it for a year or two, then changed my mind.
When I became a Buddhist, I became a vegetarian. But, sometimes I do eat meat, which tends to confuse some people. ;)
My reasoning is similar to that of the early Chinese monks, though I came up with it before hearing of theirs.
Here it is in a nutshell: I will not order meat in a restaurant or buy it in a store. I know it is already dead, but I also know that if I buy it or order it, another animal will be killed in its place. So, even if my own piece of meat comes from an already dead animal, my buying it will place an indirect request to kill.
I do eat meat without hesitation when someone offers it to me. In that case, the person has already bought the meat and indirectly ordered the killing of the next animal. That person has also already fixed the meal. My eating it or not eating it changes nothing. If I don't eat it, some other guest will eat too much and get a heart attack later, or the host will throw it out. I'd rather eat it than have it thrown out because I feel the poor animal has brought a sacrifice, involuntary as it may be, and the sacrifice would be completely wasted if the meat were thrown out. For the same reason, I sometimes even buy a hot dog, like when the local K-Mart food court is about to close and lowers the price because they'd rather sell the hot dogs for less than throw them out.
Another reason for me eating meat offered by others is that I feel I have no right to impose my beliefs on someone else. The person is actually performing an act of kindness. Who am I to judge them!
Finally, I have been occasionally getting small meat products from the local grocery store ever since I was diagnosed with diabetes. This is because these products contain virtually no carbohydrates, while vegetables have plenty of them. The first precept does not say do not kill others, it says do not kill. That implies the obligation to preserve my own life. My diabetes is the result of some past karma of mine, and as a consequence I am sometimes stuck in a dilemma. And considering the Theravada practice, I feel I am doing the best I can. And that is all I can do. Back during my Christian years I always thought I was obliged to do more than I could do, but now, as a Buddhist, I view that attitude as fallacy.
I would like to hear from other practicing Buddhists how they view the topic of vegetarianism for the dharma perspective.