If a theme park is in an area where public transport is an option, or people can get to it easily by train, or there are park and ride alternatives in the area, then fine, this gives people a choice as to if they want to to pay the parking charge or not.
Alton however are in the middle of the Staffordshire countryside. There are very poor public transport links to the park, no other places to park nearby and the roads leading to the park are narrow and in a poor state of repair in places. The parking charge was initially set up to help fund the new bypass, which, didnt happen due to problems (not Alton's doing I hasten to add), but, the parking charge has remained ever since, simply adding money to the coffers of Merlin.
Its certainly not right of Alton to have this parking charge given the new bypass is no more and the overwhelming majority of guests have no other option but to drive to the park and then get hammered not only by the entry price, but then also the car park charge afterwards, on top of fuel to get there, food, drink, souvenirs, really does leave a bad taste in peoples mouths when exiting the park after what may have been a good day on the park.
But, us Brits are a soft bunch, we think that because someone else does it, then hey ho, we have to put up with it, and that's why Alton implement it. If people stopped visiting the park or refused to pay the parking, then i'm sure those barriers would be removed quickly enough!
Until such time as there are decent public transport options to get to Alton, or a park and ride scheme set up, then the car park charge is wrong and serves absolutely no purpose other than to generate even extra profits for this greedy company.
Edit : As for where the money goes, its easy to see its not going on the roads in the park. The road down to express parking is pot hole alley and there are a lot of pot holes on the roads to the hotels. I don't see any security guards patrolling the car parks or any new security cameras watching the car parks... the ONLY thing that they do seem to have spent the money on are new car park machines and barriers!