I agree with you that this seems silly. But Microsoft itself seems to buy it. Why have they created a "mobile" interface for Windows 8 that annoys desktop users? And now Office for Linux. That's a surprise.
Windows is everywhere, but Microsoft only gets money when they sell a new version. New computers are cheaper and cheaper, so they have to lower Windows price also.
With Windows 8, it seems Microsoft itself is ready to bury the desktop. It's a tremendous mistake, but MS has a long history of completely failing to recognize trends.
The mobile market is enormous and MS missed it (so far), but the desktop/laptop market isn't going away any time soon and they seem to be missing that, too.
Windows is a good desktop environment for the most part. But by trying to make Windows a monolith on all platforms, they seem to be rushing towards making it suboptimal for all platforms and ceding a market they are the undisputed owner of. Or maybe the mass of computer users will just accept Windows 8 as the new normal and put up with it no matter how much they hate it. After all, they've been doing that with Office for 20 years.
Windows is everywhere, but Microsoft only gets money when they sell a new version. New computers are cheaper and cheaper, so they have to lower Windows price also.