Ain't no justice in DC: Law on the streets and law on the books

So I just paid forty bucks for two parking tickets.  I feel the parking tickets were unjustly imposed however for violating basic principles of ex post facto and legality, though I know that the law is a horse's ass here, and that really the principles are not strictly speaking violated.  Here's what happened.

Before I left to Israel, I parked my car in DC on the street in a perfectly good spot that required no changes to be made while I was gone.  However, while I was gone, the city's agents, in their infinite wisdom, declared those spots "emergency no parking" areas, and thus I got socked with 20 buck tickets.  Twice.

When I returned, I happened upon a cop, and asked him: officer krupke, what gives?  He said, they can do it because you can't leave your car in the same spot in DC for more than 72 hours.  But that rule is posted nowhere, I protested.  He said that's right. I then added: I've left my car in the same spot for more than 72 hours hundreds of times while I've lived in DC, never once getting a ticket or a warning.  And he said, that's right; the rule is never enforced!

I said that ain't right.  He said, This is DC. Nothing here is right.

Then I asked if there was any proof the person giving me the ticket knew that my car had been there for 72 hours.  And he said, no, none at all. 

And like a jackass, I paid forty bucks over the phone today. Argh.

If DC were run by economist types like Rosenberg and Shavell, of course, they would simply ticket every luxury car, frequently for no good reason, and they would collect extortion payments from people like me who haven't the inclination or time to fight these scoundrels.  But there's probably something illegal about that...