Why don't you charge drivers of motorcycles with no or ineffective mufflers? It's probably one of the most asked questions I hear after complaints about the new gaseous discharge headlamps. Fortunately, it's one of annoyance rather than safety.
The situation of no muffler at all is probably the easiest to deal with. A quick examination of the pipe will reveal no obstruction and a ticket can be issued. The explanation in court is a simple one and the traffic court justice is assured of the situation making a conviction more easily obtained.
Having an inadequate muffler is where the difficulty lies. While the Regulations are clear that the opinion of the inspector is sufficient, the justice is not so easily assured and without an independent witness or a measurement with a decibel meter a conviction is not nearly as easily obtained.
The next problem is that decibel levels are specified for an inspection facility, not the side of the highway where the police operate. The levels would serve as a guideline but are not definitive. In addition, I have yet to learn of a traffic unit that had a decibel meter of their own.
Having a loud exhaust to some riders is a lifestyle issue. Regardless of the action that the police might take, the exhaust is not going to be repaired, or will be repaired long enough to pass inspection and then put back the way it was in the first instance.
Oh, and for the record, the police DO deal with motorcycles that have loud exhaust systems.