From a business point of view this makes sense: it's safe to assume that WP7 sales are nowhere near the sales of Android or the iPhone. At the same time, it's a safe bet that the number of of Windows7 machines deployed already dwarfs the entire Mac user base (and that's not counting the number of Macs that run Windows pretty much full time). So if you were Balmer, wouldn't you try to take as much of that influence with you?
Whether this is best for consumers or Microsoft I'm not sure. We shall wait and see.
The assumption here is that Windows 7 machines will continue to dwarf Macs and more importantly phones and tablets that unhitch users from the Windows world. Sure PCs will be with us for quite some time but their cash cow is going to be giving quite a bit less milk in the future IMO. Balmer would rather rest on Microsoft's laurals just like Bill did with the Internet. When people think of the Internet they immediatly think of Microsoft right? No, I'm afraid pr0n has one once again...