Keep the Peace
The other issue is that often, when people encounter other rude passengers, it becomes difficult for them to stay calm. Those who can remain peaceful help keep the environment manageable for all, but this is not the case for most wronged passengers. In fact, who amongst us has not encountered a very uncomfortable experience while on public transportation?
Looking for an Argument
Even if you do remain calm, sometimes there are passengers that are just looking to antagonize others in order to get into a fight. Sometimes, these fights can even become physical, which is really never the answer, and is certainly not worth it just for a seat on public transportation.