I agree, in general my personal belief is we should be totally free to do pretty much anything that doesn't infringe on anyone else's safety or freedom.
With regards to motorcycles, I don't mind a nice sounding stock bike, but the ones that are ear-splittingly loud are a menace. The worst I ever heard was in front of a starbucks a few years back, I was on a lunch break with a friend and a guy hopped on what looked to be a pretty late model sportster 1200, and he wailed on the throttle and it literally shook the windows. Definitely *far* more than stock noise level. I had a ringing in my ears for the rest of the day along with a splitting headache. I really hate to tell others what to do, even less that the gov't should tell them what to do, but that's just excessive. Even a switchable exhaust setup would be preferable, something the rider could switch over to when he thinks it's appropriate, but not in the city where he can cause actual physical pain to bystanders. It's akin to someone playing a high power custom stereo at over 100 decidels, only it's on all_the_time. All of the riders I've ever known personally were far more responsible and courteous than to run that kind of setup, but the relative few that do kind of ruin the image for everyone else.