ZebuSanctuary, at one point that may have been the case.
My SIL used to have a prepayment meter, I think for the reason you mentioned in your post. I'm not entirely sure how much she was in arrears, however I do know that they were taking back an unlawful amount each time she topped up the meter. I looked after their house when they went on holiday and they had left me £20 to top up the meter. I thought, "£20? It doesn't need that much, I'll just put £10 on and spend the other £10 on a bottle of vodka". Oh what a mistake that was. I put the £10 on and it was immediately taken. Every time they topped up, they'd have to add an extra £10 to the amount.
I have since been told that the energy company shouldn't have done that, that they are only allowed to take a certain percentage, and that percentage isn't 50%.
It is one of the reasons why I don't like prepayment meters. Also, if your electric/gas run out at 2am (this has happened to my mate on more than one occasion) and for what ever reason you can't top up the meter online, you're screwed until you can get to the shop in the morning.