Exactly.... I read in the Off Road mags about this very topic and they went over the specific rules and laws of every state and California is the only one which has numbers and decibel levels to cross the line into illegal. Every other state goes by what that Cop's opinion at that moment if he thinks it's too loud.
I drove a 71' Chevy 3/4 ton Flatbed on 38's full time my last 3 years of High School. Had a pi$$ed off built 283, Long Tube Headers with corners and tips that came out right behind the front tires. Got pulled over for muddy windows a few times and for having more than 4 forward facing white lights on during oncoming traffic. Never anything about exhaust. That was Alaska though... While stationed in WA for 2 years and San Diego for 3 years Also had a Subaru GL on 31's with True Straight Duals that came out right before the rear tires. Only got pulled over for having no mud flaps, no front plate, no light on the rear plate and for standing water sloshing around half way up my tail lights the cop said was a fire hazard. Never mentioned exhaust. Also in San Diego I built an S-10 with a screamin 409ci sbc, headers and straight pipes that came out the sides behind the rear tires. Drove it everyday for 2 years. Got pulled over for my HID's blinding him as he passed me and for my stock bumper mounted tow ball covering up my rear plate. Never mentioned exhaust... I've had many many other vehicles and none of them had cats or mufflers after the first few days in my garage. I've also never got pulled over in them....
My Aveo has a hollow cat and no muffler, stock pipes. The O2 sensor in upstream of the cat so the computer doesn't realize it's not there. Just lets the motor exhale easier. Over a year in San Diego, the drive home and almost 2 years in Alaska. Never seen flashers in my mirrors..... I also started recording better Fuel Millage numbers after I fixed the exhaust and it sounds pretty throaty when I step on it and roll down my windows so I can hear it...