Actually, the law could be more than just compliance with Federal regs, so it's cause for concern if it complicates adding after-market exhausts.
Without seeing laws finer details, I'm not sure what the Denver law means by stamp. Is it a stamp you get at the DMV after passing a noise test or do the after-market exhaust manufacturers have to add the stamp at their end? If the latter, if you want after-market pipes in Denver you're screwed, because few, if any, exhaust manufacturers are going to make special Denver-only stamped pipes. If just a DMV sound check is required for the stamp that's more rational.
No other US cities require a special stamp to my knowledge, as it's usually up to an officer's discretion as to what it too loud based on Federal guidelines and common sense. Seems to me Denver's perhaps approaching the problem with red tape that won't stop modders, but will put more cash in the city's pocket and not necessarily save the ears of its citizens. After all, the noise violation is already enforceable without the new law if I'm understanding the Federal regs.
Case in point: I find it odd that it's $500 for a first time offense -- more than a speeding ticket or other lesser traffic ticket that actually endangers lives -- so the law seems dead set on killing after-market exhausts on all bikes completely and generating revenue for the city.
Mind you, I was a noise offender way back with riding a 2-stroke RD350 with seriously loud pipes (back in the '80s). I got pulled over and given a "fix it" ticket in SF. I did put the stock pipes back on and the cops were happy. Even with inflation it was no where near the ludicrous $500 first offense fine. That's just absurdly expensive. Ironically, I now ride a WeeStrom with a stock pipe, so it's quiet as church mouse and most of the time I prefer it -- I do miss the sound of that roaring Muzzy on my old ZX9R though.