It's a mixture
Many differing factors on this one really.
Firstly, I have been playing computer games for nigh on 25 years now. I remember as a young'un playing on the Atari 2600. Pong, Space Invaders, and some game with "planes" and "tanks" in where you tried to destroy the opposition. All blocky, vector graphics agreed. Still playing games today, yes you notice the realism now. Playing the likes of Forza 2 and GT5 makes my mouth water, but it does not encourage me to rag my motor round the streets Fast and the Furious style! But even back when I had my Sega Master System with Shinobi, and Golden Axe, and Double Dragon (I could go on) did I ever think "Hmmm, I really must go out a beat the seven bells of **** out of someone". To be fair, people being idiots and ignorant in real life is more than enough incentive to want to dish a bit of pain out now and again, but obviously we don't do that, we know right from wrong.
Secondly, as has been mentioned, parents not wanting their kids around while they watch Jeremy Kyle, Eastenders, Corrie, whatever and so ship little Johnny off to his room, with the latest game he pester powered them into buying. "So long as he's not bothering us, he's doing alright". Their own parents never spent time with them as kids, so they in turn do not really know how to be a role model and parent to their kids. And guess what is going to happen to little Johnny when he hits 14 / 15 and becomes a dad to baby Shaniqua? He's not going to have the first clue how to be a responsible parent (whatever age he is) as he was never exposed to it himself. It's a continual cycle due to a lack of responsibility on behalf of the parents.
Not all parents who buy <insert favourite 18 / 16+ rated game here> for their kids are irresponsible as such, but I do agree that their own lack of knowledge about the core fundamentals of such a game leads them ignorantly into game shops to buy it for little Johnny because he nags. As was said above, if they sat there and watched their child play these adult rated games, you know within minutes a half responsible parent will do the right thing, take the flak from the child and get rid of the game. It's the "parents" who will more than likely sit with their child and play also, encouraging them to rob cars, run people over, bludgeon them to death with a hammer. These are the children at most risk of having a distorted sense of reality as they see their "role model" going "Yeah, get him, Do him over. Keep kicking him now he's down! That's how you do it!"
Heck, in an ironic sort of way, games like GTA, CoD4 etc allow me to vent my frustrations in a safe environment as I know the difference between reality and fantasy, and they allow me that avenue to get pent up rage out of my system on a computer sprite rather than losing the plot in real life (Falling Down with Mikey Douglas?).
Some parents need educating and you'll see them suddenly take more notice. They then will make the call if their child is mature enough to play a game rated higher than their age. Some parents will never care so long as it keeps the child happy and they are not being pestered. These are the future "issues" that will come back to bite us.
Paris, because she has probably never been told "No" by mummy and daddy in her life!