That's because the American rating system is stupid. Here in the UK, most films rated R get 15 or 18 ratings, which means you have to be at least that old to get in. Everyone knows children will sneak in one or two years underage, but you don't get small children in films that blatantly aren't' for them. That's also why we don't get stupid condescending censorship cuts for films aimed at adults when they feature frank sexual content, nudity, etc (apparently in America, violence is fine, but sex is a no-no, which is why films featuring the latter often get snipped to get an R-rating - complete madness in films aimed at adults).

I'll never forget during my first ever visit to America (in early 1998) going to see a horror film and seeing parents in the row in front of me who'd dragged along their three-year old. He alternated between bored and terrified, and I felt very sorry for him. I wish they'd taken him to the rerelease of The Jungle Book (which was playing on the screen next to ours).