In many places the police have no authority to issue a ticket for something occuring on private property. For parking, many times the LEO has to see the offense and the owner or manager of the property has to sign the ticket. Simple signs with "damage at owner's expense" language makes things simple for when the tow truck has to drag it out of the parking spot to haul it off and hooks to the flimsy bumper first...... oops.
If it's someone that does it chronically, just call the cops once and have them provide the verbal warning to the owner so that there is a valid record and then have it roughly towed away.