I wonder if this has much/anything to do with Valve's Linux push?
I know that it is possible to do Kinect stuff on Linux, but if they can make it easier on Windows then perhaps they can get people building Kinect games that target Windows rather than aiming for cross platform?
Technically, you could always develop for Windows using OpenNI however you'd typically be relying on a 3rd party wrapper to expose the bindings in .NET. I'm not sure if this library is utilizing OpenNI or has been built from the ground up, but regardless: it does encourage developers to use their library rather than a separately maintained one.
In saying that: OpenNI works pretty well with the Kinect if you spend some time understanding it! I've never used the Windows bindings however on OSX it works well :)
I know that it is possible to do Kinect stuff on Linux, but if they can make it easier on Windows then perhaps they can get people building Kinect games that target Windows rather than aiming for cross platform?