I think this is often a case of people dealing with cognitive dissonance.

It's pretty easy to understand that in at least North America our livestock handling practices cause a huge amount of suffering to animals for the enjoyment of people. But most people are able to ignore / justify that with a "well, it's bad but you gotta eat".

When presented with a group of people who make the choice not to eat animals / contribute to that suffering it challenges that justification and makes people very uncomfortable and in some cases angry. The easiest way to resolve this conflict is to mock those people and write them off as some kind of sanctimonious kill joys