Probably because they think one is worse than the other. I think flipping somebody off and hitting them in the face is both not very nice, but I think one of them is much more impolite. Moral judgements come in degrees. Is this novel information for you?
That said, maybe they're also trying to minimise the impact of the plant based foods they eat? I'm sure you don't, why go halfsies and all, so that thought probably never crossed your mind.
You believe that eating animals is morally wrong, but you want to pretend that you're eating animals, at the expense of non-animals?
No, they like the taste. They're not pretending to eat meat. People who drink diet coke don't pretend to drink non-diet coke. They're drinking another coke flavored beverage.