i did a debate on this very topic, and if you look at the research it does make sense to impose some sort of fine against retailers who sell M-rated games to people under 17.
one senator (you might remember him as al gore's nomination for vice-president) referred to the ESRB rating system as one of the best rating systems in any industry.
there is statistical evidence that violent video games do in fact cause some sort of psychological damage to children (all they've really proven is that it slightly inhibits the ability to learn math in young children), but all of these are very minor and the impact is not proven to be any worse than when watching a violent movie or television show.
but, then again, movies have a rating system that is enforced--somewhat--effectively. several large retailers already enforce the ESRB rating system in their stores: Wal-Mart, Sam's Club, Sears, Montgomery Ward, etc. Many other stores are beginning to enforce the system: Electronics Boutique, Best Buy, and whatnot.
The argument that it is "unconstitutional" and that kids that buy M-rated games do so with their parents knowledge. This is bunk; and even if it wasn't it would be a moot point. I myself own many M-rated games: Unreal Tournament, Max Payne, Quake I-III, Soldier of Fortune I and II, Half-Life, and others. My parents, however, thing I waste hours away on my computer playing Homeworld over and over and over. They have no knowledge of what games I play, and have not been able to prevent me from buying any of them.
Even if the law were passed there is nothing saying that a parent cannot go and purchase an M-rated game and allow his child to play it; just as a parent can go and purchase an R-rated movie and allow his child to watch it. The bill only regulates retailers, it doesn't invade into the household.
I, personally, think it makes sense to enforce the ESRB rating system with fines against stores that sell M-rated games to minors. My debate class agreed with my partner and I unanimously, as well.