Disabled parking placard tiff

[fotofashion]

The smoke is still coming out of my ears a bit after an unpleasant experience this morning. Scene: WalMart parking lot, disabled parking spot. DH left me off at the front door so I would not have to walk far and went to park the car. He found a spot in the disabled area, parked, put the sunscreen in the windshield and hung the placard, as per instructions on the placard, on the mirror facing backward (so it can be read from the rear window). When we came out, we had a ticket for no placard! Well, the "person" who was handing out the tickets (and had given one to the person in the spot next to us, too), was still there so I went and knocked on her window and showed her I indeed did have my placard and it had been displayed correctly. "Oh", she says, "it wasn't visible" and showed me a picture she had taken from the front of the car. I tried to show her what the placard instructs one to do but she kept insisting that the placard wasn't visible! I got mad and just threw the ticket into her lap and told her I would not accept the ticket and left.
One thing I am wondering, are the police allowed to give infractions on private property like that? In any case I will not pay the $20.00 fine. Someone needs to learn the rules regarding the use of the placards. I assume federal rules supersede any local ones in this regard.
Thanks for letting me vent.
Beverly A.