The point of this type of play is to reinforce societies values. Children want to be the Hero so they emulate the qualities of the Hero. In many modern video games the protagonist is an Anti-Hero. GTA is a perfect example. I won't buy it for my kids because I don't think that a game that rewards players for theft, blackmail, racketeering, murder and selling drugs is appropriate.
Now I recognize that the current rating system places far more emphasis on sex and violence than the actual subject matter, but fortunately most developers that think anti-social behavior is cool also think graphic violence is cool. This means that most of the games I would object to get rated as Mature. There are some that get rated as Mature that I don't object to, but then I can buy them for my kids if I choose to do so.
The line that gets parroted a lot is that it's the parent's responsibility to keep their kids from buying these games. That's a bunch of crap. It's not unreasonable for a 13 year old to spend a Saturday afternoon with his friends playing stickball in the abandoned lot or watching a matinee at the cinema. You know what else isn't unreasonable? It isn't unreasonable to expect salespeople not to sell an M rated game to a bunch of kids that decided to pool their money and buy Grand Theft Auto 27 instead of going to the theatre.
I find the idea that allowing children to play amongst themselves for a few hours without parents hovering over their every move or sending them to convenience store for some milk is somehow shirking parental responsibilities to be ludicrous. Part of rearing a child is teaching them independence. That requires doing things on their own without a parent present. It is the responsibility of other adults to step in and moderate unsafe behavior when children are out of the immediate supervision of their parents.
We shouldn't need a law to make it clear that it might be a bad idea to sell a product that is clearly marked "Mature 17+" to children that are obviously under 17. The mere fact that it even has such a label might indicate that there may be a substantial number of parents that may object to the sale of said product to their child. Unfortunately since many people either can't grasp such a simple concept or just don't care, legislation may be the only answer.
If self-enforcement actually works, then we don't need legislation. I don't call children being able to purchase age inappropriate material 42% of the time working.