A car doesn't have that little registration sticker on its license plate. A quick radio check, though, shows that the vehicle is properly registered.
It's possible that the sticker was stolen. That is a problem, especially as car registration fees keep climbing and fewer people can or will pay the amount due.
It's possible that the sticker just fell off.
If you were a parking checker, what would you do? In this case, the parking checker had the car towed.
Maybe it was a bad day and the parking checker was tired and crabby. Maybe the parking checker just has too much power and could not resist using it.
Towing seems like an overly harsh penalty because a little sticker isn't on a license plate. Maybe it's time for the city to revisit this one and differentiate between unregistered cars and cars that are registered but aren't wearing their sticker.