Why they would care? Who knows.
It could be because a set-top box can be easily connected to a VCR or DVD recorder, which almost everyone has, whereas much fewer people have decent video-capture on their PC, and YouTube keeps going out of their way to try and prevent their content from being downloaded.
The “premium” providers tend to be the ones to mark all their content as not playable on a set-top box (eg. the videos from the “official” Iron Maiden channel).
Copyright reasons and distributability seems to be a likely candidate.
