The violence isssue will never make a law until they can legitmatly prove it otherwise in a relevant scientific study.
Storeowners hesitate to not give a younger customer a M rated game cuz they could lose the customer or have another one come along and say something like, "you let him get one but not me?" and before you know it you're in some stupid sueing situation over a stupid $50 game.
Someone sued the creators of Mortal Kombat in the early 90s, and lost because they had no proof the violence in the game caused the kid to go crazy.
It like how smoking was in the 70s and 60s. Store owners didn't care if you bought cigarettes under age, everyone was doing it, but until research proved that cigarette smoke can cause cancer and kill you, the government did little to enforce the law. Is the violence issue really any different? not really.