The American MPA is self-regulated (probably simply to put it into legal territory, since legislating it would probably prevent freedom of expression and translates to censorship, especially if the authority is directly government-controlled) but some other systems are legislated (not just in countries you think as opressive, mind you: Britain has an independent organization that rates movies but are approved by the government).

Still a bit flawed though: I thought that there is already a ratings board in America for software?