Like I said earlier - I think people are pissed at the underhandedness of it all. If they had just increased the price of the pass and kept it simple, yes there would have been complaints, but not this huge backlash!
Merlin think people are idiots, and won't work out that it's basically forcing you to buy a premium pass. I feel sorry for them if they are making a loss, and if they justifiably said, look we've invested a lot in our parks over the last few years and we are raising prices accordingly, I for one would just accept it and get over it.
Also, it's their most loyal of customers that this affects the most - those who will be racking up charges of hundreds of pounds a year in parking fees!