@SteveRichter: Because this model allows them to make far much more money.
The desktop is now seen as a mistake in its business model, as it allows companies too much freedom, see here and here
Basically, moving on from now, Microsoft will own at least 20% to 30% of everything, and will make it difficult for people not to adopt the Apple model.
This is good news if you are a shareholder with MSFT of course, and the letter posted to above is what any company that exists to make money should do and has done.
Microsoft will be pushing for all their server infrastructure to be cloud based, and specifically Azure based, which is where all their best people have gone including Guthrie and Zander, and their finest lawyers will be working to find a way to get businesses to move to Azure.
In a way, Windows as it stands now, and in the future will be just a thin client to Azure, and I think they will move to a cheap or even free version of Windows (upgrades are available now for $40) where they will make most of their revenue from Azure subscriptions from all their home users. No need to worry about ever losing any data as your desktop, tablet and phone will all be connected to Azure. It is just too large a market for them to ignore and will allow them unprecedented growth.