I answer is that do it because they are allowed to do it.
The whole purpose of the privatisation of essential services was to allow private companies to make huge profits, and that is what happened. Another reason that the Tory government of the time brought about the legislation was to allow a small number of ordinary people to make a very small killing on the Stock Market, for the first time. The idea was that people would be so glad of this opportunity, that they would vote Tory next time, and it worked, at least until Major ran out of steam in 1997.
There is no reason whatsoever why anybody should pay different rates for the energy they use, based on their preferred method of paying. When I go to Tesco and buy my groceries, I am not charged a different price if I decide to pay with cash, a debit card or a credit card. But Tesco would charge different rates, if they thought they could get away with it !
Whether some people are finding it difficult to pay for their energy is another topic altogether. Maybe some way could be found to take the energy costs directly of the benefits received. But to charge more, to the very people that find it difficult to pay, for essential energy is immoral.
I doubt whether this will be popular with our right-wingers but there it is !