I do think that MS THINKS that tying D3D to Windows version will sell more games. But to be honest I think for the most part it just slows adoption of the newer APIs by devs. D3D11 is really nice, and can support a huge range of hardware, and his been around a long time, but only recently has it become 'the standard'. These antics by MS do more harm to them in the long run IMO.
This is my opinion as well, and I think that strategy likely ends up forcing many people away from directx entirely. If directx 12 turns out to be exclusive to windows 8+ then it’ll be useless to me as a developer. I’m still using windows 7 and so is a large portion of the market, so no matter what improvements they add in 12, it’ll be a choice between abandoning a really significant percentage of potential users or sticking with an earlier version of directx or OpenGL. (not a hard choice)