Little Tommy may play the game and see some blood or boobs. Imagine the horror!
Also, morons buy M rated games for their kids all the time despite being warned at Gamestop that it isn't for kids. I've read a few stories about this. Your kid likes the cover art and you don't feel like dealing with a tantrum ten minutes before closing. So you buy an inappropriate game you are warned about. Then you are appalled when said game isn't for children. It's a game right? A toy!
It's no different than when you buy your kid a DVD with an R rating then have a fit because people are naked and being murdered.
We have to think of the children.