No one has mentioned one of the real reasons those bikes are so loud: So people in cars know they're there! As we all know, bikes, (bicycles and motorcycles), tend to be "invisible" to auto drivers. Lights help, but "cagers" sometimes miss those, too. Some drivers "target fixate" on lights, but will steer clear of loud noises. So motorcyclists use what they have, (their exhaust note), to alert cars around them to their presence, since they don't have the luxury of special lanes for them. That way when a car does hit them they have to say, "oh I didn't see OR hear him, your honor". Yeah right.

Motorcycists have a saying: Loud pipes save lives. Although I really don't like all that noise spoiling my serenity, I tend to agree with them.