For some reason unknown to me, Thorpe's target audience is young, social media addicted hooligans who like to swear alot, smoke weed, and use words like "gassed" to explain their excitement.
The MAP targets families. Families aren't welcome at Thorpe Park. To lesser extent, it seems they're not particularly that welcome at Towers these days either.
The money is made from families, and most park operators here and abroad seem to understand that. Therefore I find the whole MAP holder model quite bizarre. They discount entry to their parks by selling passes for cheap to families, whilst downgrading the experiences in the parks for entry paying guests who they make all the money from.
So it shouldn't be a surprise really that Chessington is rammed with passholders spending bugger all on secondary items, whilst Thorpe is full of disrespectful teenagers who tuck their trousers into their socks.
Also worthy of note is the fact that Thorpe is the only Merlin park in the UK that still sells edible fast food through Burger King and KFC. Make of that what you will, but I suspect that kids who've already fleeced their middle class dad for the entry ticket won't tolerate extortionate prices for dog crap in a roll, whereas the Alton and Chessington passholders will just bring a backpack full of crisps and sausage rolls.