In layman's terms, a cycle with even straight pipes can roar like a dragon when the throttle is wide open, or it can purr along disturbing no one if you're gentle with the throttle.
Unfortunately, there are way too many riders out there who feel the need to make their bikes 'roar like a dragon' when in residential neighborhoods. Since the biker community has done nothing to police itself, it is not surprising that laws are going to be used to crack down on the noise.
There are a two guys in my neighborhood that love riding around in low gear at high rpm. One is a dual-sport and the other a hardley with open pipes. I have talked to both of them about it, and they both say they should not do it, but they continue to make pests out of themselves. I don't think it should be a law enforcement issue, but they are asking for it.