Some are, some aren't. From the Theravada perspective, the choice of whether or not to eat meat is purely a matter of personal preference. Many Buddhists (and, of course, non-Buddhists) do eventually lose their appetite for meat out of compassion for the welfare of other living creatures.
Although the first of the five precepts, the basic code of ethical conduct for all practicing Buddhists, calls upon followers to refrain from intentional acts of killing, it does not address the consumption of flesh from animals that are already dead. Theravada monks, however, are clearly forbidden to eat meat from a few specific kinds of animals, (flesh of humans, elephants, horses, dogs, snakes, lions, tigers, leopards, bears, hyenas, and panthers) but for reasons not directly related to the ethics of killing.
Monks are to pursue vegetarianism by leaving uneaten any meat that may have been placed in the alms bowl, but because they depend on the open-handed generosity of lay supporters (who may or may not themselves be vegetarian) it is considered unseemly for them to make special food requests. In those parts of the world (including wide areas of south Asia) where vegetarianism is uncommon and many dishes are prepared in a meat or fish broth, vegetarian monks would soon face a simple choice: eat meat or starve. Taking part in killing for food is definitely incompatible with the first precept, and should be a voided. This includes hunting, fishing, trapping, butchering, steaming live clams, eating live raw oysters, etc.
But what if I eat or just purchase meat: aren't I simply encouraging someone else to do the killing for me? How can letting someone else do the "dirty work" possibly be consistent with the Buddhist principles of compassion and non-harming, a cornerstone of right resolve?
The Dhammapada expresses this sentiment succinctly:
All tremble at the rod. All hold their life dear. Drawing the parallel to yourself, Neither kill nor get others to kill.
Dhammapada V 130
Clearly we should not intentionally ask someone to kill for us as when, for example, we order fresh boiled lobster from the restaurant menu. But purchasing a piece of meat from an animal that was previously killed is another matter. Although my purchase may indeed help keep the butcher or restaurateur in business, I am not asking him to kill on my behalf. Whether he kills another cow tomorrow is his choice, not mine. This is a difficult but important point, one that reveals the fundamental distinction between personal choices (choices aimed at altering my own behaviour) and economic political ones (those aimed at altering others' behaviour). Each of us must discover for ourselves where lies the boundary between the two. It is crucial to remember that the Buddha's teachings are, first and foremost, tools to help us learn to make good personal choices (kamma); they are not prescriptions for commanding action.
We are all guilty of complicity, in one way or another and to varying degrees, in the harming and death of other creatures. Whether we are carnivore, vegan or something in between, no matter how carefully we choose our food, somewhere back along the chain of food production and preparation, killing took palce. No matter how carefully we trod, with every step countless insects, mites, and other creatures inadvertently perish under our feet. This is just the nature of our world. It is only when we escape altogether from the round of birth and death, when we enter into the final liberation of Nibbana the Deathless, can we wash our hearts clean, once and for all, of killing and death. To steer us towards that lofty goal, the Buddha gave us very realistic advice: he didn't ask us to become vegetarian; he asked us to observe the precepts. For many of us, this is challenge enough. This is where we begin.