This is a by-product, I understand, of legislation designed to stop people in rented accomodation being overcharged for electricity.
The previous method on sites was to charge a fee for the pitch and a supplement for electicity.
Due to the regs, which came in a few years ago, this is prohibited. They can include it in the price of a pitch, but the only way to charge seperately would be to put a meter on each pitch, which, with the waterproof housings etc. would be far too expensive.
Now that the majority of units want hook-up (including trailer tents and even pure tents)the cost could not be justified.
I believe it may theoretically be possible, by disconnecting the supply to a pitch, to redesignate it as "non-supplied", but I am not aware of anybody who has actually done this.
I guess if you are going to be charged, you might as well get a lead and use it!