[zmmmmm]

Unfortunately, many people are letting their kids (young teens, pre-teens) do that. (Where "letting them" simply means, not actively stopping them, being unaware etc).

Which is half the problem. The other half of the problem is that Facebook is actively pushing developers to social-enable their games by integrating Facebook based social features into them (hence the mandatory Facebook account). So even games that have no reason to be using those features are seeing them added.