There have been decibel level requirements on motorcycles in California for decades. That said, there is another part of the law here that says that "any exhaust system that is not stock on a bike, or is modified from stock, that makes it louder than it was when sold new is illegal". Now, that being said, I run loud pipes here in California and so do a lot (and I do mean A LOT!!!) of other bikes are louder than mine are. The LEOs don't pay much attention to larger bikes with loud exhausts. I even know of a few cops (and some of them are motorcycle cops) that also have personal Harleys and they too have modified (and loud) pipes. As long as you are not one of those A$$holes that sit at lights and just rev and rev just to make yourself obnoxious, there should not be any problem.

We should all read this.

The California Air Resources Board has fined Santa Fe Springs-based Vance & Hines Performance LLC $500,000 for violations of air quality laws related to the sale of illegal aftermarket motorcycle exhaust systems in the state.
ARB said a late 2012 investigation found that Vance & Hines sold, advertised and offered for sale motorcycle exhaust systems that replaced original emissions-critical components, such as catalytic converters.
California law prohibits the sale, distribution and installation of parts that modify vehicle emissions control systems, unless modifications are proven to neither reduce the effectiveness of those systems nor cause excess emissions.
State officials said Vance & Hines fully cooperated with ARB in the investigation and has implemented a compliance plan for its aftermarket systems.
Funds from the fine will go to the California Air Pollution Control Fund to support air quality research.

And V&H is not the only one.