Parking is a service, provided by the city. The people who pay taxes have already paid for that parking spot. If you, or somebody else (Even the city!) try to profiteer in excess of that, it's predatory and against the rules.

If this was in a private parking lot; knock yourself out. Charge whatever you want. Charge a hundred dollars a minute. But it's public space, and you cannot hold public property hostage and expect me to pay. You would be no better than a group of highwaymen holding up a bridge for a "crossing toll". It's not yours to profit from!