Aftermarket exhausts are already illegal to sell or install in California if they are manufactured for, or installed, on any 2013-year motorcycle or later model, unless it has been "certified" by either the motorcycle manufacturer for that model or approved by the CA EPA (which is looking for a lot of paperwork).
You won't see a dealer installing a such a muffler as they can get hit by big fines should they get caught. It just isn't worth the risk. However, that law doesn't seem to prevent some "cruiser" or sportbike owners from replacing their mufflers with items bought out of state (for off-road use, of course
) or just removing them entirely like the ones that ride by my home do.
You can make the laws, but if no one enforces them you are only wasting your time and trying to feel good. It is a political thing....