Sharraxaadda

There are signs posted up and down the street that there is no parking of trucks or commercial vehicles. There is a cab driver who lives on the odd side who parks his cab (or two) and never gets a ticket. Calls to the police get the response, "Cabs are not commercial vehicles." Excuse me, but, WHAT?? How is a cab NOT a commercial vehicle?
How is it that a MINIVAN with a company name on it is considered a commercial vehicle, but a cab with a company name is not? What's the difference?