I think it's probably because it's unintentional, unavoidable, and you're given a big warning painted on the back of a truck to tell you that driving too closely will damage your vehicle. I'm just speculating though, there may be more to it.
we can all look back and tell Blu that he was acting childish and obscene, and, well, he was acting childish and obscene, even dangerous by cutting off a bike. Hindsight is 20/20, and in any case, Blu probably realized he met stupidity with more stupidity whenever the situation had already passed.
But presented with the same situation in reality, how many of us can honestly and rationally say that they wouldn't have gotten angry and possibly done something similar? Anger, rage and aggression are human nature, and no, they aren't rational, nor can you rationalize it. Anger basically impairs your ability to think rationally, and you're basically running off primal instinct. It basically goes like:
Little bitch pissed me off->I got mad and reacted->jeez, I should have thought about this before I did it.
Given my temper, I'm sure I might have done something stupid/dangerous in retaliation as well if I was put in that situation. And after the fact I might have realized it, and the last thing I would want to hear from anybody else is "how could you do something so stupid?"--mainly because 1.) I don't know how I could have done something so stupid; nobody actually thinks when they're angry/threatened 2.) I've probably already been asking myself that repeatedly; is you getting on to me really that necessary?