Why is what diet someone chooses to follow, veganism, vegetarianism, being a total carnivore or just your regular "meat and potatoes" kind of person matters to anyone else. Nothing needs to stop except criticizing people for their chosen diet.
Some people argue for the ethics, ie whether it is humane or not in today's modern society or for some made up religious reasons, etc.
I was arguing with a vegan a while back saying it's OK to eat animals for sustenance. By the end of it, I was labeled more fit to rot in hell than the worst of the worst. Well, plants don't want to be eaten either, some have evolved for defensive mechanisms just for such threats. In a perfect universe, we wouldn't need to consume other living things to carry on. We were placed here without a real choice, so we should all get over it.