Second, she seems to implicitly rely on the premise that whatever is customary in a society is moral. She points out that all societies make somewhat arbitrary distinctions between the things they eat and the things they don't eat, and based on this, she declares that all such choices are merely a matter of custom. But not all customs are moral. Slavery was deeply embedded in the fabric of the American South, but that certainly did not make slavery moral. The moral vegetarian is precisely asking us to reconsider whether our deeply held customs are moral. (I'm not saying that eating meat is morally comparable to slavery. I'm just using slavery to illustrate the logical point.)
Ms. Niman appears to hold that while it would be wrong for us to eat dog meat based on custom, it is proper for the Chinese to do so. However, whether it is moral to butcher a creature should not depend on where it happens to live. Let me use a fanciful example to illustrate this logical point. Imagine the world is taken over by many warring tribes. Some eat human flesh; some not. Whether or not it is permissible to kill and eat you shouldn't depend on who your tribal overlords happen to be!
Finally, Ms. Niman seems to hold that psychological attachment is key for determining morality. It is the fact that we are deeply psychologically attached to dogs and not to pigs that permits us to eat pigs but not dogs. However, imagine an infant orphan that no one loved or cared about. Surely that would not permit us to make infant orphan sandwiches! The moral vegetarian would argue that the fact that we don't care about pigs the way we do about dogs is not an excuse to eat pigs but a moral failing on our part.
If not psychological attachment or usefulness down on the farm, what then is key for determining how we should treat a being? Ms. Niman is aware of some compelling answers—namely intelligence, self-awareness, and the capacity to feel pain—but dismisses them as irrelevant.
However, most people (when not contemplating a tasty pork chop) will agree that such factors are all that could be relevant. After all, why do we believe it is okay to kill a cockroach but not a human being if not because the cockroach is far less aware and intelligent than we are? And, to invoke another fanciful example, why do we empathize with Avatar's Na'vi if not because of their intelligence and capacity to feel pain?
Ms. Niman admits that dogs and pigs are comparable on these attributes. Therefore, she is wrong and the vegetarians right. Either you deem it acceptable to eat both dogs and pigs or you deem it acceptable to eat neither.