You've never seen people throwing red paint on people in the streets for wearing leather or fur. You've obviously never seen vegans take over a whole section of a grocery store to protest meat.
We can't argue politics nor religion, so this is just one area allowed, where we can argue, banter, and build some comradery over a split issue. Vegans are just an easier target, because they'll make themselves known, and it is obviously a decision, so it's not like we are making fun of something someone can't help.