Yup, back when they were the only real option, they could get away with it.
But now you can easily get a Freesat+ box (or a Freeview+ box), I don't see how Sky can continue to justify charging* to use PVR functions...
*[those with no subscription]
As for the Sky+ fee itself though... yeah, it used to be the case that it was only free if you had a Premium channel (Sport/Movies), but they actually
dropped that nearly two years ago & changed it so that Sky+ functions were free if you had *any* Sky subscription... even just basic Mixes.