AndyC seems to be worried that someone would fork the library as a GPL library and make it better then Microsoft's own implementation. However one thing he is missing is that someone could make a GPL implementation and it could very well be better then Microsoft's implementation using the specs. I guess Microsoft missed an important loophole.
Ok, so you still don't get it. Let's try again:
If a reference implementation falls under a GPL license it becomes unusable to anyone not using the GPL. This makes it useless as a reference implementation.
Hence it needs to be protected against that and released under a more permissive license. Which is what Microsoft have done.
That's why the BSD license would be an ideal license.