You do realise that the space for feeds inside satellites isn't unlimited? Each feed is independent, whilst it can hold a lot, I don't understand how you could think not charging for it is sensible?
They gave an additional service and you expect them to give it away for free?
No you misunderstand, the feeds exist regardless of whether the boxes have the ability to record and an extra coax cable running to them, the satellites don't provide a feed for every box, they provide hundreds of channels over a few sats all the time, your box either records these or not, the costs are the same for sky.
It's like saying an extra signal has to be sent when you record something from freeview on your vcr, it's not, you're simply recording whats always there.
It's not a additional service, it's a function of the hardware.