From my experience, though I'm not a parent, I've noticed a lot of people just don't want to be bothered. When I worked with the public, we would try to let parents know if their children were doing dangerous things or if they were being a bothersome and a lot of parents were annoyed that they were approached.

How often have you been on a plane or something (sorry, this is my scope of existence) and a kid is kicking a seat or playing with the tray table and the parents do nothing? I realize that there is a difference between annoyances and danger but it is the same obliviousness going on.

The really scary thing is when you can't even tell who the parents are! Sheesh, I look after my bike or cats better than that