Many US states keep laws like this simply because they're vague. That makes it almost impossible for the average person to fight the ticket, so most simply cough up the fine- which is what the court wants, in the first place.
Mufflers have nothing to do with emissions, although pretty much everywhere you'll find laws stating that it's illegal to run without them.
Here in CA, it's highly unusual to get pulled over JUST for sound. Generally, excessive noise is something you're cited for after being stopped. Our 95db (not 93 as someone else posted) is very generous, and in the first couple of years of that law, over 90% of cited vehicles were found to be legal. Most other states/counties/cities aren't as lucky and have those "illegal to modify" laws still on the books.
i think the hondas and acuras get f***** with the most. my buddy had a 93? sentra ser with a loud muffler and he got pulled over at least twice with that muffler in 2 years or less owning the car. probably the cops were profiling his car. he lived in the bay