Microsoft has contributed/released Apache licensed code before (even GPL code before).
I don't think the API of libfreenect is all that complicated. It has wrappers to a bunch of different languages (including C#), I doubt Microsoft will support Java or Python while libfreenect does.
If Microsoft likes it or not, a lot of hackers and academics use Linux for their research. And you know Microsoft would never support Linux for anything interesting, so there will always be a niche for libfreenect no matter how "great" Microsoft's own API will be.
I should say, the way the write this, it sounds like they are only releasing a driver for Kinect. _IF_ they release the computer vision middleware as well then it's a whole different story.
You say that this is pointless, then ramble on about Linux. Well, "whether you like it or not", there are lots of researchers and hobbyists that don't use Linux. I'm a hobbyist myself, and don't touch Linux and want nothing to do with Linux, and this Windows Kinect SDK would be great for me, so it's cearly not "pointless". Linux geeks think that they are the center of the universe and everything is about them. Well, not everything is about them, and whenever a software kit is released it should not be dismissed as "pointless" just because that kit doesn't cater to the Linux crowd.