As metalblue rightly says, that's a myth. If the Private Parking Company (PPC) has displayed suitably prominent signage clearly setting out their terms, you are assumed to have entered into a 'contract' with them by parking there. In order for a PPC to bring a case to court, they simply have to demonstrate the existence of such a contract, that you breached the terms and that they incurred a loss as a consequence of your breach. Remember also that these are not criminal but civil matters which are decided on a lower burden of proof, that being the balance of probability. Regardless, it's a high risk strategy to simply ignore a penalty.