I think where this law intends to go is this:
Johnny, 13, goes and buys a game that is rated MA. Assuming a law is passed that makes it illegal for anyone under 17 to buy this game, his mom/dad finds out and gets very upset. 30 min later mom/dad comes storming into the video game store and demands to know why they sold their kid a MA rated video game. After venting for a few min they leave and go to a lawyer and demand the store be fined for breaking the law, same process a store would get for selling alcohol to minors.
So is this law, if enforced, going to make children less violent or protect them in some way? Can you also argue that keeping kids from seeing R rated movies makes them less violent and protects them? We already restrict access to movies and we have violence in schools that is appauling.