The best reason is that kids do very dumb things, and they take risks, without thinking through the consequences. The explosion in children exposing themselves, and getting entrapped by pedophiles into exposing themselves, not to mention the things they might say and do for attention, surely warrants some kind of monitoring. As a kid I had zero policing of my internet usage, perhaps that's why as a parent I will monitor and limit my children's access and usage. tl;dr the risks are too great not to police at least a little