They are policed. Brian Crecente reports on multiplayer online games and the occasional criminals who play them:
Modern massively multiplayer online games don't have to deal with just cheating and player hazing, but gold running, character theft, even real world crimes, like players targeting underage players for real-world sex or discussing crimes they want to or already have committed. ...
... if a player is signed in as an 8-year-old boy, but the program believes they are chatting or behaving like a 40-year-old, moderators will be alerted to watch them.