Because you cannot hear the exhaust whereas I can (and my exhaust is still all OEM parts), it makes it a subjective comparison and is the basis of the OP's quandry. To make it objective, a side by side test with a dB meter is required.
While an aftermarket exhaust can amplify the exhaust volume (or even just removing the rear res), if done correctly it can also keep the volume at or near to OEM levels.
On edit: By allowing subjectivity to drive a punishment, the law itself is flawed. People hear differently. This can be caused by genetics, environmental influence, accident, whatever. It places burden of proof on the accused (guilty until proven innocent) rather than on the accuser (innocent until proven guilty).
On edit part 2: Several states adopted
this model initially, as it provided a hard and fast limit to exhaust noise. Over time though, states have been moving away from this and going with a "no louder than stock". What defines louder than stock? At what RPM? From what distance? At what speed? Compared to a vehicle from where? What if you live in the rust belt and had to replace the exhaust due to environmental decay... you are not modifying it. See the problem with the law?
This is the exact text for Washington state (previously used the 95dB law):
http://apps.leg.wa.gov/rcw/default.aspx?cite=46.37.390
(3) 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. A court may dismiss an infraction notice for a violation of this subsection if there is reasonable grounds to believe that the vehicle was not operated in violation of this subsection.
The loophole is "muffler originally installed on the vehicle". What is to say that by swapping the OEM muffler with a magnaflow doesn't change the volume? Since the law is implying using your vehicle alone as the point of comparison, if the original muffler has been discarded there is nothing to compare against.
Keep in mind... I am not a lawyer....