This seems to be the law in Washington state:

No person shall modify the exhaust system of a motor vehicle in a manner which will amplify or increase the noise emitted by the engine of such vehicle above that emitted by the muffler originally installed on the vehicle, and it shall be unlawful for any person to operate a motor vehicle not equipped as required by this subsection, or which has been amplified as prohibited by this subsection.

http://apps.leg.wa.gov/RCW/default.as…

So... basically every aftermarket exhaust is technically illegal here?