My guess is there may be inconsistent policies between different content providers, with some disallowing forward-skip and some allowing full control. I don't use the DVR function a whole lot so haven't really tried to connect the dots.
That would be kind of weird, if only because you can still skip ahead, you just have to pause it and then use the scrubber to move forward instead of being able to do it in 15 second increments. I'll have to see if I have that issue with any other video playback apps.