The vegetarians argue that the problem with eating animals is that they feel pain. We know that this is true of plants. They just don't care about plants. They need to make an argument on the premise "plants are morally worthless because they are plants", which would be true, not on the premise "plants are morally worthless because they don't feel pain", which is false.

People abstain from murder (and rape, though that's much less analogous to the dietary debate) because society sanctions murder and rape, and where it doesn't, because the victims and their families are likely to take exception. (Compare traditional Icelandic law, where killing a person was legal under a variety of circumstances, but concealing the fact that you'd done so was criminal. Consider that the United States fully criminalized rape within marriage in 1993.)

The second of those reasons does not, and will never, apply to food animals. They have no power to take retribution. The first one doesn't, but can; it is the reason Brahmins eat a strict vegetarian diet. But it is not particularly compelling; very few people are impressed by the argument "you should abstain from meat because it makes me happy when I can make other people obey my commands".

And that's what leaves moral vegetarians in the ridiculous position of arguing from known-false premises. It doesn't mean I can't laugh at them for trying.