Actually its a well known psychological phenomenon where if there is other people around to deal with a problem most people will allow those other people to deal with the problem.
Strangely enough if ONE person would've stopped to check on him, more would have.
The "exceptional" people are the ones that actually stop to help. What those other people did was normal. They are cool as long as the situation doesn't affect them. And involving themselves in it would've affected them.
There are numerous psychological studies throughout out history that help describe this phenomenon, but its been so long since psychology 101, I can't remember any of them.
