I presume that there is a barrier which opens when you take a ticket and then enter the car park. My contention would be that I have not 'parked' until you have actually seen the prices to be charged and effectively agreed to them by leaving the site. If you do not agree to the proposed charges then you should be able to leave the car park without paying anything. The timescale that would allow you to do this would be of the order of a 'few' minutes, any longer then you may be deemed to have accepted the terms and conditions and be liable to pay.
No criminal law would be applicable, but that of contract law