Standardized DRM, afaik, isn't finalized in any part of HTML5 yet (I could be wrong about this). I would assume that their agreements with the studios require them to use at least some form of DRM on all the videos Apple hosts but doesn't own (which includes trailers). An easy way to see if they have some form of DRM is by trying to take a screenshot of a playing video. If it has DRM, the image will have... a big hole where the video should be*.
Furthermore, from what I've gathered, it seems as if the video files Apple has come straight from the studios, and are only stored on Apple's server. In other words, they probably don't have the ability to abruptly change formats without writing up new contracts. Even more so, I not sure that Apple even has the ability to convert the files even if they wanted to.
* Oddly enough, this doesn't seem to affect the screenshots taken on iOS, even with videos purchased from iTunes.