I think its yet another example of how Microsoft's relationship with the Mac simply hasn't changed since the first one (or even the Lisa) in the early 80s. The Mac has generally always been a better user experience, and Microsoft's Mac team has often been called the "R&D Lab" for future Windows versions, both in the OS and in the core Office app.

I.E., on the other hand, was one of those foot-in-the-door strategic moves. It wasn't to really support the Mac as much as it was yet another blow against Netscape. Microsoft never put their full resources into making it the best browser for the Mac, or else it would have had more feature support on par with the Windows version. When $150 million bought desktop dominance of IE (a cheap bargain, if you ask me), Netscape's final nail was set; the Mac was a tool against Netscape, not an end in and of itself, in that instance.

IE on the Mac was never a "how do we do this well so we can make the Windows version better" the way that the Office for Mac is viewed.

Now that Apple is making a headstrong commitment to having the "best" browser for the Mac, and IE seems destined to being strongly tied into the next-gen .NET (which will go hand-in-glove with the next Windows and likely not portable to the Mac at all, continued support of IE for Mac is just wasted dollars.

Better now to drop the support and hope a little FUD of "we're not supporting a soon to be dead platform" comes out of it, leading more end-users back or over to Windows. Won't work, of course (Jobs is smarter than that), but its how Microsoft thinks.