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 runningcharacter 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.

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