I've said this before on Thorpe Park Mania forum, but the parking charges are
The reason for this (in addition to law covering the area), I found out after having the same issue happen to me as people have described. I had paid the parking fee, yet at the end of the rather busy day, the barriers were raised to ease traffic issues. Upon complaining through email, I received a response which was mostly sympathetic about the issue, but did honestly state that the charges were opt-in, therefore no vehicle leaving without paying the charge could be seized or fined.
I have visited Thorpe four times since that incident and have never paid the charge, only once have I had to explain myself at the barrier to a member of staff; the other times I have been let through without question.
I say this as a fan of Merlin parks and a ten year annual pass holder, but I will not be charged when the park's poorly designed infrastructure has, in the past, had me waiting 90mins to leave the car park, all to profit from stealth charges
shame on you Merlin.