There are NO laws forbidding a retailer from selling an M rated game to children (unless that game falls under existing pornography laws). They have tried to make laws like that but they have been ruled unconstitutional every single time.

Hmm, you're right about that. I apologize. I misinterpreted some information. I went and double-checked myself, and you're correct.
The ESRB will fine a game publisher for sending an incomplete game in order to get a lower rating, but can't do anything to a retailer.
However, many retailers are imposing programs of their own to prevent the sale of M rated games to children. GameStop's policy:
http://www.destructoid.com/gamestop-sell-an-m-rated-game-to -a-minor-enjoy-unemployment-29690.phtml
I was worked at a Toys R Us, and I was told that the store would get fined for selling M titles to children. I assumed it would be a government fine, but it may have been corporate cracking down on individual store managers.