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.
> I'm a sado-vegan. I only eat plants that have been proven to actively signal other plants when being killed.
I've never understood why moral vegetarians seem to think that hurting and killing plants is so much better than hurting and killing fleas. Plants are well-established to react to injuries and to communicate to other nearby plants when they're under attack. If you're going to argue that you have a moral obligation not to hurt chickens, you need to at least be ready to argue that you don't have the same obligation not to hurt onions, which feel strongly enough about being attacked that they actively fight back.