There's a big difference between not going out of your way to support something and going out of your way to prevent it. Windows doesn't have a native POSIX interface (it used to have a basic one) but you can add one if you like. It can be done higher level via something like Cygwin, or it can be done directly in the executive just like the Win32/64 APIs. There is nothing stopping you from adding it, they don't care.
Same deal with DirectX and OpenGL. A Windows GPU driver has to provide DirectX support. It is just part of the WDDM driver. Windows provides no OpenGL acceleration, and no software emulation. However you can provide your own OpenGL driver if you wish, and Intel, nVidia, and AMD all elect to do so. Windows does nothing to stop this and they work great (if the company writes a good driver). Indeed you could develop your own graphic API and implement that, if you wished.
There's a big difference between saying "We aren't going to do any work to support your stuff," and saying "We are going to work to make sure your stuff can't be supported."