"Scofflaw Harley riders get ignored, free to continue assaulting the ears of innocent Americans everywhere."

I've always wanted to know, too. Why do they have to be THAT loud? Some of them are fairly quiet, and then a phalanx of distortion comes past.

It's a quality of life issue for those within earshot, and a quality of life issue for the dutifully costumed riders. But perhaps when everything is a quality of life issue, nothing is a quality of life issue.

The Unbearable Loudness of Harley Hogs: Why are they allowed to violate noise ordinances? No one seems to know, by Bruce Murphy (Urban Milwaukee)

... Harley-Davidson representatives always tell the media that they respect noise ordinances and don’t favor violating the law. But… when Harleys are sold, they emit 80 decibels, which is already loud (about the sound equivalent of a lawn mower) and arguably in violation of the city’s night time noise restrictions (which state that “no power tools or equipment… may be operated between 9 p.m. and 8 a.m.”). But their design allows owners to easily modify the cycles with “straight pipes” that don’t mute the noise and exceed 100 decibels; some have estimated the noise exceeds 115 decibels.
And most bikes get modified.