You should have asked him to show you the state law that provides for that.
Here in Az, officers generally carry a big, thick book with all the statutes. If they accuse you, just ask them to point out which law it is you broke, and tell them to show you in writing.
I once had a new DPS officer here that pulled me over in a F550 dump truck. He tried to get me for no seat belt, but the law at the time said I didn't have to wear it in vehicles older than 1972...it was a 1970 dump truck.
Then he tried to get me for no CDL, but the truck was registered at 26,000 lbs. I only needed a CDL if it was registered for over 26,000 lbs.
So then he tried to tell me I needed a health card with my class 2 license for a vehicle over 21,000 lbs, so I asked him to show me that law, since I was unfamiliar with it.
He couldn't find it, and couldn't issue a citation on it because it didn't exist.
He went over the vehicle thoroughly, and finally killed the battery by the time he was done. I had to get a jump start from him, but he didn't give me any tickets.
This happened in the early 1990's, and I've done the same thing a few more times since. LEO's are often mistaken in their understanding of the law, so when they're wrong it'd good to let them know...in a kind way, if possible.
Daryl