Just one example
See Fred Rader's car here -http://looserocker.com/?page_id=27
Read about his dealings with Illinos, and a few other things here -http://classracer.com/classforum/showthread.php?s=2888f3c2e4c0f0fa38e237cb87af261c&t=53250
There is other info out there, just not finding it. Bottom line was that Fred was let go at the time, then received a letter from the state DMV with a ticket and a $750 fine for being commercial without a CDL. Except he wasn't commercial in any way, shape or form. So do not cound on the state to be the "final" answer.
As you noted, the cop that stopped you makes the rules, doesn't matter what the person you talked to at State or DMV said on the phone. It can well be BS but you have to take it to court to prove it.