Did you know even vegetarians aren't supposed to eat parmesan cheese?
There's an enzyme mix called rennet that was traditionally harvested from cow/ruminant stomachs.
Now that we understand how all the microbiology works, it's actually cheaper to grow the bacterial cultures that produce it instead of harvesting the traditional way, but it's actually EU law that Parmigiana-Reggiano (the real stuff) must use rennet for some reason.
I just think that's neat.
Also, dumb law, but I guess it's nice they tried to care about food quality.
Of course, if you're just buying something labeled parmesan in America it's probably fine, because no one cares.