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.