Today you, tomorrow me 
This actually makes much more sense than a rich family stopping. I think its got to do with our psychology. We support each other because one day we'll need the support ourselves.The richer folk see less need in stopping to help because they feel like they're self sufficient; in other words, if they themselves run into car in the future, AAA and cash will bail them out.This more humble family however knew that if something like this occurred to them, they would definitely appreciate the help. Thus they were more inclined to stop. So I don't know why people here were so surprised...Hell, the guy himself states all I just said in four simple words. Still, it was quite a touching story.