This is why I'm glad that my house on campus has a private lot in the back just for our house. Since it's just us, there is no towing contract for it. If we see a car that isn't ours, then we can call a tow truck if we want, but I'll usually wait a while in case it's someone who was just leaving it there for a few minutes.
But yes, towing companies could care less about if you're parked in a legal spot or not, especially those working on commission. I'm lucky enough to have never dealt with any of these crooks.