Why do they make the law about motos? Cages are often loud too. So are leaf-blowers, jackhammers, fireworks, and a bunch of other crap that people probably shouldn't be messing with during quiet hours.
I lived right along the Kennedy expressway in Chicago for many years, out towards O'Hare airport. With the windows closed, you could barely hear planes, el trains, or traffic going past...until some pinhead with a ridiculous stereo or set of open pipes went past. It didn't matter if they were on a bike or in a cage.
Now, I'm not the sharpest tool in the outhouse, but if I can ascertain that noise is noise, regardless of what it's coming from, why can't legislators and LEOs, ie., supposedly intelligent people, ascertain the same?