The only thing I can figure is this:
It is impossible to completely lock down audio with DRM. There a ton of utilities (and even old fashioned analog ways) to make perfect (or near perfect) copies of audio recordings without "breaking" the DRM.
DRM video on the other hand is a little easier to control. To the best of my knowledge, there is no easy way to make a lossless (or near lossless) copy of a DRMd video through analog or digital means, and they probably want to keep it that way.
It also occurs to me that with all the anti-trust stuff going on in the EU regarding Apples "monopoly" in the digital music realm, they can use the QTKit as an argument that they provide an "open" way to enable playback of purchased audio.
Conversely, Apple doesn't have a portable video stranglehold, so it's ok (from a litigation liability standpoint) to keep the video more restrictive.
Of course, this is all wild speculation
While I don't know what the effort would be like, perhaps rather than quietly removing DRMd video, perhaps it could get a little lock icon or the like, so that people understand that the video is there, it's just not playable. Maybe add a pref to hide "locked" video.
(At the rate I am throwing in my two cents, I am going to be a poor man!)