Same reason you don't get a ticket with a modified exhaust. It is up to the cop....

 

police can pull you over for anything that makes the rear tags harder to read. i've been pulled over at night in rockville cuz the reflective paint chipped off around the numbers and the officer couldn't run my plates. he showed me where it was chipped off, told me to go to mva to swap plates, no repair order, then after i got back in the car, asked for my license and registration so he could run those but i've also seen people "racing" around here without any plates or without using headlights late at night to avoid getting caught, so i figure the police are just trying to do what they can.

to the OP - i asked a friend who is a police officer about aftermarket mods, but i never got a straight answer. all i was told was that anything that isn't oem/oem replacement has potential to cause you problems. anything that looks, sounds, or even smells off can be a potential reason to pull someone over or give you a repair order. and if you really piss them off, you have potential to get repair orders for anything and everything cuz it might be broken. just another plug to be courteous to the police. who knows, maybe they really only wanted to check out your car?

just have to remember that police are just people doing their job, and sometimes there's not much for them to do. one time an officer told me and some friends to "get some alcohol; it's been a slow night..."