Or, more food for thought:
Why can African Americans make fun of white people, and it's all cool, but then white people make fun of African Americans, and we're racist?
The world is just like this. I think it plays on history: your grandparents most likely ate meat, and perhaps they are still alive and healthy. And since they did, then we can to, after all, it goes way, way back.
Just like the black/white thing, except for with this one you have to change one thing: people have changed! Since we no-good whities who personally owned slaves on our family farm were so racist, and now that's over, then the african americans who remember when they were our slaves, and who have the beating-marks to show it, can be racist to us.
If history repeats it self like this, then indead, we will have permission to openly make fun of meat eaters.