sysdharma recounts an experience with kids throwing stones at each other and parents not intervening, supposedly based on the notion of children having to push back to others by themselves, if they don't like it, otherwise they would not learn to become independent. Whatever the cultural context is, I believe this idea is BS. Kids can only handle problems up to a certain level; above that, they need assistance by parents.
We also tell our children they must say "Stop!" if they don't like what another one is doing. But after the third time, I would certainly intervene, because I don't want to teach my kids they should start throwing punches if the other does not listen (except if they are directly and physically attacked with threats of serious harm, i.e in self-defence situations.)
I have some suggestions for such situations (please note I do not abhor sarcasm).
sysdharma has absolutely the right (bad) feelings about the incident, in my opinion! thumbs-up