I haven't read all the 'hey idiot', 'idiot on this street', 'idiot at this place' threads as they all seem to be pointless. Actually, this thread is pointless too. I'm at work and I'm bored. I'm gonna take a break and write a little something.
I realize people get frustrated when they see others doing something that seems dangerous, or perhaps something they wouldn't do themselves. All I have to say to this is, just let it go. If you really feel the need to express your opinions and feelings, maybe you could say something like 'it was not cool what you did, you could have hurt some people, and most importantly, yourself'. What's the point of ragging on someone for doing what the hell they feel like doing, it's their life.
Granted, if someone does something that directly affects you, ie passing you in your lane, cutting you off, etc. then yes, voice it. I don't remember who it was, but someone stated that we all break the law (speeding, lane splitting, whatever), it's easy to do on a bike.
Don't get your panties all up in a twist, chill.
P.S Don't start flaming away, yes, this is only a suggestion, if you want to swear and rant and b*tch about something you saw that you didn't like, do it. Good for you, for recognizing something that falls under your definition of 'dangerous' and 'stupid'. Congrats.