One other factor not mentioned here is that a great number of those running very loud pipes (Mostly HD riders around here) feel obligated to rev the engine whenever the bike is running but not moving, on start up, stoplights, crosswalks, in the driveway, the drive through at the bank/fast food joint etc. etc.
Now my BUBs are pretty loud when I jump on it, but I make a point to keep as quiet as possible when in residential areas.
