Rock, Paper, Shotgun has an interesting article on Battlefield 3 that explains why you can't shoot civilians in the game. Apparently EA decided to make it so that players couldn't just gun down innocents in the game. It's an interesting policy considering that some might consider taking away that option as removing some of the realism that games such as Battlefield 3 and Call of Duty Modern Warfare 3 are promising.
The game’s executive producer, Patrick Bach, said that players often want to engage in provocative behavior:
“In a game where it’s more authentic, when you have a gun in your hand and a child in front of you what would happen? Well the player would probably shoot that child,” he told RPS.
While the choice - if it was in the game - to kill the civilians would be the player's Bach also worries that his company and his product will ultimately take to blame for it in the media.
“We would be the ones to be blamed. We have to build our experiences so we don’t put the player in experiences where they can do bad things,” he added.
Bach admits that there is a degree of self-censoring in the game to limit player behavior.
"Me personally, I’m trying to stay away from civilians in games like BF because I think people will do bad. I don’t want to see videos on the internet where people shoot civilians. That’s something I will sanitise by removing that feature from the game."
Bach added that the games he makes are for grown-ups and that he thinks "games need to grow up a bit." Perhaps shooting a civilian in the face is a bit childish, but it's also harmless fun because his game is rated "M" for mature and therefore not for children in the first place.