Maybe it is more a local custom than a tourist trap.
I arrived to Asilah by own car at the end of September. There were many free parking places near the city walls of the medina. When I was parking a car I noticed an Arab man running towards my car. He had a plastic bag in a hand, and he put out a cap from the bag fastly. He immediately weared the cap with inscription PARKING GUARD. Although his clothes were not too clean he was very polite. He said hello or bounjour and indicated with his finger the inscription in his cap.
I said OK, watch my car but he wanted me to pay immediately. I (as always in Morocco) said OK but not now. I will pay when I will come back. And that man got a bit ungry but finally said OK.
When I came back (about 4-5 hours later) he was sleeping on the grass in a shadow of a tree but he immediately awake to get money from me haha :-). Usually such local guard costs 1-2$.
Unique Suggestions: Take it easy, keep smiling and accept it, please. These guards are often very, very poor. You can give them (extra or instead of money) a small gift, for example a pen, a pencil, a cigarette etc.
Fun Alternatives: You can escape but I do NOT recommend it.
We met a local on the train -- he seemed nice (of course), and told us we should go to Asilah with him, since he was getting off there to go shopping for a gift for his sister's wedding. He said we could tag along for the ride and that we could leave our backpacks at this lady's house who rents out rooms for artists during peak season, and for a small amount of money, she would feed us. We asked him, how much he thought it would be, and he said probably around 10-15 bucks.
We went, they fed us lunch and dinner, the local boy that lived there showed us around (which was great!), the guy from the train took us to the medina and souqs, but when it came the time for us to leave, he charged us more than double!!
The money wasn't the issue, it was just that he was being so deceitful... If he told us how much it would cost in the beginning, we wouldn't have any problems with it... call it naivete or stupidity, but I like to believe that not everyone is out there to get you... =P