I don't know, it's kind of a tough issue. On one hand, I'm not so worried about the violence as much as the adult subject matter/disturbing images etc. in M rated games that children would be poor equipped to understand without parental supervision. So no, I really don't think that they should have access to them without parental guidance as a rule.
However, by enforcing and regulating rules regarding the purchase of these games -- based on ratings that are supposed to be suggestions rather than hard rules -- it's just encouraging parents to be more and more lazy in the raising of their children. Why talk to your kids or try to be a part of their lives if the government will watch them for you, even though the government does a shittier job than the parents? It seems like a really popular trend for parents to dump their kids off on the school system, television, video games etc. rather than trying to take an active role in their lives. The only thing that fining stores for selling M rated games to minors would do, would be to make it easier for parents to ignore their children's lives to an even greater extent. It would be just another safety net for bad parenting.
What we need to be doing is emphasizing personal responsibility rather than enforcing rules and regulations regarding media of any form. If your child brings home an M rated game, offensive music, R rated movies or what-have-you, the parents should be the ones to blame for failing to notice, and set appropriate rules and boundaries regarding such media. This is NOT a public health issue, it's a domestic issue, and the government has no place in it whatsoever.