Its considered federal safety equipment. Much like the cat converter its only mandatory where testing is done, but anywhere on a public highway if a cop happens to somehow figure out you don't have one they can write you up... We all takes our chances.
I have had a lifetime of bad luck on the weird crap, I once got written up for non DOT seat belts in a car that didn't come with seat belts! I put Simpson belts in a 65 Baja Bug... I got a ticket for transporting alcohol within reach of the driver I had a case of beer bungeed on the luggage rack of my MGB,,,, I've gotten a speeding ticket on a bicycle 50 in a 35.. (Ok I deserved that one...
) I got a ticket for missing wipers, while driving a jeep with the windshield taken off.. One fo rdriving too slow on a highway,, The bicycle ticket was the only one that cost me money...
At one time I had a easter basket full of fixits and non moving violation citations, most collected in my late teens when I owned a screaming 60 hp Datsun 1200 cc pickup.. Painted Chrysler Corona Taxi Yellow... Would just barely hold 70 on the freeway if I had a hill to get up to speed.
Cheers...
Dave