It's better than letting those other two guys get picked on, IMO. According to the comment posted above, the incident had been going on for around five minutes and no one else was stepping in to do anything.
Could be because it was no one elses' business. If the two that were involved didn't want to at least try to take care of it, why was it this guy's business? The short answer-it wasn't-I hope he goes to jail.