Violent Games Cause Violent Reactions?

I came across a great video that discusses the violent video game issue. The main cause for everyone talking about these games as a bad influence, is honestly the real life kids who do in fact do something stupid from seeing a game. These games do have a rating scale, so isn't it up to the parents to watch what their kids play? I mean how can a 12 year old buy a M rated game with out their parents.