As usual, I was right and you bone-heads were wrong
I've looked in the DOT Regulations in Parts 393.209 (steering system) and 393.3 (additional equipment & accessories) and I cannot find where steering wheel spinners are illegal or unapproved.
The only way steering wheel spinners (knobs) would be illegal, is if its done so by State Law in the state you are stopped in.
Thanks for the question. Hope this helps.
Sr. Trooper Monty Dial
Texas Highway Patrol
Commercial Vehicle Enforcement Division
Garland, TX 75043