Many people here have a common sense understanding of tort law. However, that does not negate assessing liability.
So for example, instead of parking, let us use a more visual example to better explain the situation.
Suppose neighbor A posts a garage sale sign on neighbor B's house while neighbor B is on vacation. Neighbor A then breaks into neighbor B's house and conducts the garage sale. Customers participating in the garage sale have a reasonable expectation that the person conducting the sale is the owner of the house.
Several days later, Neighbor B returns home to find his household items gone!!!
Of course, neighbor A has broken the law in conducting the garage sale under false pretenses. I don't think anyone would argue with that. However, what about the customers? What liability do they have?
The law in this case would suggest that despite expectations that the person conducting the sale was the owner of the items, the end result would be the items purchased were still classified as stolen. So, while the customers may avoid obstruction charges due to "reasonable expectations", their possession of those items may be forfeited. In this case, they would have to pursue legal action against the neighbor who conducted the sale in order to obtain compensation for their purchases.
So, as it relates to the parking case: Yes, these people who had their car towed were deceived. However, this does not absolve them from parking on private property. The individuals who were towed would have to pursue a legal claim against the party which falsely erected the sign to recover their fees. Hope this makes sense.