They already know that people who "shouldn't" be playing their game are playing it. I'll bet some of the devs kids who are under 17 have played them. The target audience is middle school to high school. And yes, bad parenting seems to abound in todays society. It's partly why we end up with such a large quantity of absolutely useless people.
Back onto the subject though, the people the game is targeted too(in my experience) don't really care about story or emotional depth, they just like to shoot things. So the level is not really necessary, as the people who want to play the game just want to shoot things, regardless of whether they are terrorists or civilians. I wish it wasn't so, but the vast majority of people like this have made it too hard to deny that any longer. I'm not saying that EVERYONE who buys these games is like this, just most of the ones I've met.