The main reason a vehicle is towed when it's parked in a unauthorized area or "illegally" parked it's usually because there is a law or city ordinance that forbids the parking of any vehicle in the area. There are usually signs or some type of marker to inform you if you can or can't park in the area. It's not a random act with it has been made clear that you are not allowed to park in the place that you parked. 

If you park illegally get ready that your car will be tow sooner or later.

Usually a vehicle will be towed more quickly if it's parked in an unauthorized area and obstructing traffic.