Actually this is a very logical decision for Microsoft to take.
Consider that Microsoft's Office business is actually a bigger business than Windows. Now keep in mind that Windows is slowly moving from holding a position of total dominance of all personal computing, toward being just one of several aspects of personal computing that now also encompasses smartphones and tablets.
Given that the vast majority of those smartphone's and tablets are running iOS or Android, not you start to see how many of Microsoft's customers -especially business customers- are going to be clambering for a cross-platform office suite.
Holding onto the hope that Windows will prevail could instead have resulted in Office being dragged down too. Microsoft are NOT going to let that happen, hence we see Office for iPad today, and we'll likely see Office for Android tablets coming soon too
Of course we'll also see Office 'touch' versions popping up in the Windows 8 store too at some point. Microsoft now need to work hard to highlight the other advantages that their platform does still have over the competition. Native multi-user support, native side-by-side multitasking, better file and network browsing, as well as host of business features including Active Directory integration (at least for Windows 8 Pro devices).
The fight goes on, but Microsoft are protecting their other business units with this move.