"Loud Pipes Save Lives" - my favorite non-humorous bumper-sticker.
First off, while emission standards may be tied to the Feds, noise levels are decided at the state / local level. And each state has its own acceptability level for 'cycles. Grossly violating these standards is a good reason to get pulled over - not much fun regardless of the legality of your bike. But the acceptable noise standards are typically higher than for cars.
The noise standard is generally different for bikes than it is for cars, because many state legislators have been convinced of the arguments posted above. Loud pipes really do save lives. Just about all of the close calls I have had on my bike over the past several years have been caused by other drivers not noticing me. As was stated above, the smaller profile of a bike renders it harder to see. When someone changes lanes, pulls out of a driveway, or whatever, the quick glance they make (if that) can easily miss me. In addition, the sleeker profile (the bike, not mine!) makes distances and rates of speed harder to judge. The sound of an oncoming bike makes my presence much more known, and thereby safer.
Keeping in mind that a small fender-bender between two cars can be fatal when a 'cycle is involved, I’d like to apologize for annoying you. [melodramatic]I’d rather do this than ask for your apology for killing me.[/melodramatic]
On a last note - the above arguments hold no water for defending those chuckleheads who insist on gunning their engines, revving up, etc. at three am. You can shoot at them.
Once in a while you can get shown the light
in the strangest of places
if you look at it right…