Why ban social networks?

As mentioned in my previous post, banning social networks in schools and libraries, I wonder why? Is it to do with our fears brought in with a neo-con government in the states creating false fears within society? Is it our culture within society, which from books like lolita in the 1950's to the present day film called Hard Candy [a film dealing with online stalking]? Is it that parents now leave there kids at home without supervision to surf the web? Its these three in my opinion.
We need to teach kids life skills against paedophiles, like not giving out addresses or phone numbers. Teaching them that the world is not a pure place all the time. But we don't need to make them scared, just aware of the dangers that are there. Wired also provides an interesting article on how adults can see how kids see the net on the virtual mystery tour. So come on. Kids have to learn about the world, but they also need some independence.