Because the law is, ostensibly, to keep children from buying violent video games. Except something like half (there is a study on this, I can try and dig it up if you want) of violent video game sales where the recipient is a young child are done by parents. If parents don't want their kids playing violent video games, they need to be educated about the games rather than just buy whatever and then bitch that the state isn't doing something that is their responsibility if it's their concern.