I think the thing that gets me is the attitude, and Martyn B has just kind of nudged it in my head.
If you think about the way the Merlin parks are presented, they're a kind of "must do" day out. The UK theme park equivalents of a summer blockbuster. Going on a single theme park trip this year? It's likely to be at a Merlin park.
As such, you resign yourself to the fact the day is going to be full of hidden expense.
Are Drayton (and Flamingoland, et al ) really that comfortable in their ability to draw in customers for their parks that they can afford to add these hidden expenses? I honestly don't know, but I think there's a degree of arrogance in adding these charges and I don't know if the smaller parks have the back end to support that.
On the "pay on entry" thing Marc, Drayton is a nightmare to get into at the best of times. I remember they did pay in your car one year (at the entrance booths), and the queue was backed up right to A5. Took over an hour to get from the roundabout up to pay. Dreadful start to your day and they just can't cope with it.
I do agree with the idea, but the infrastructure just isn't there for it.