This is the first time I feel like I understand Microsoft's motives in the Windows 8 fiasco. Until I'd read your comment I'd been thinking along the lines of: Microsoft doesn't want to pay for long-term development of both PC and Phone operating systems, so to save long-term development costs, they're creating an OS that will work on both hardware platforms. That line of thought never really made sense to me because Microsoft makes money only on Windows and Office, but they continually spend huge amounts developing other code that has never and probably will never make them any money. Why would they suddenly become so stingy when it comes to making separate user interfaces for PC and Phone, especially when they are risking alienating virtually their entire customer base?
Now I get it. This isn't about coercing customers into using a cheaply-developed UI, to save Microsoft money. It's about (possibly illegally) leveraging the Windows monopoly to coerce independent developers into creating Phone-compatible apps - thereby potentially expanding Microsoft's monopoly into another line of business. It's all about "developers, developers, developers!" and to hell with "users, users, users". Ballmer is betting his company on this - and possibly risking another antitrust action.