The flipside to that is that what if the parent recognizes that their child is mature enough to realize that just because you can press a few buttons and an animated gun will fire in a game doesn't make it okay to do in real life? Once your child can realize that running off a cliff like Wyle. E. Coyote chasing the road runner isn't a good idea, they're taking a step in the right direction.
It's also the reason why a lot of parents who do let their kids play games like this ease the kids into the genre. Hell, I had a chat with a father while I was at work today about this very subject, and he had his son with him, who must have been no more than 7 or 8. The kid was getting a Turtle Beach headset, and I asked him what kind of games he plays. "Halo mostly" he said. I looked to the dad and said that that's a good choice of game, because it has a stronger dissociation from reality than other games like CoD, BF, and GTA because you're fighting aliens and mutant aliens. He agreed with me 100% and then went on to say he's amazed how violent some of the other games are. My little brother, who is 13 now and plays pretty much anything started the same way - around 6 or 7 he started playing Halo: Combat Evolved. He's since moved on to CoD, GTA, and BF, but still plays all the Halo games.
If you want them taxed, that's fine. But I sure as hell don't want to suffer because other parents are letting little 5 year old Jimmy run around stabbing hookers to death in GTA then blowing a cops head off with a shotgun 30 seconds later. Very similar to why (at least as a Verizon corporate rep told us) most wireless providers have moved to mandatory data packages on almost all of their phones - "Little Sarah is 10 and got her first phone and went and downloaded $300 worth of programs on her phone and now the parents are encountering recurring subscription fees as well as going data charges, and we are tired of saying 'Okay, we'll waive the fees this time ONLY'" all because the parents are fucking retards who need to pull their heads out of their asses. I didn't get my first phone till I was 17. My little brother won't get his till he is 17 either. Yet we still suffer for having to pay for mandatory data we don't use because other parents are fucking idiots.
If you can't be a responsible and reasonable parent, please, feel free to remove your gonads and spare us all another worthless swimmer in the gene pool we all reside in.