This isn't necessarily someone not knowing something... just a guy completely failing to think through what he's doing.
I bought a car several years ago. I bought it used private party, so there was no dealer tag or anything. In my state one has 7 days to pay the taxes and get a license tag for a new car; there are no other requirements. I carried the bill of sale and all the paperwork in the front seat in case I had any issues.
On the third day I had it (and the first time I drove it anywhere other than home for fear of this situation - I bought on a weekend), I went up to the tax collector's office to get my car registered after taking care of all the paperwork. As I was waiting on an opening to turn into the parking lot, I see blue lights behind me. I pulled into the parking lot across the street and stopped for the officer. He asked why I don't have a tag, and I tell him that I had just bought it a couple of days ago and was going to register it now, here's the paperwork showing the date and I'm within the 7 days, etc. He proceeds to go on a long rant about how he keeps having to waste his time stopping people like me who just can't be bothered to fulfill their legal obligations.. yada yada yada. I just stared open-mouthed and when he finished, pointed out (possibly foolishly) that yes, I'm aware I need to register my car, that's why I was trying to turn into the tax collector's office. He stopped, told me to get it taken care of and have a nice day. I can only assume he was having a very very bad day. The whole situation with the place and everything was beyond comical. I mean really, someone demonstrates to you that they are not, in fact, doing anything wrong while they are actively fixing this perceived wrong, and then you continue yelling at them about how they're completely irresponsible and doing something wrong? My head really had trouble grasping that one. I hope his day got better.