Is this something new? Microsoft has almost always supported popular platforms other than their own if they think they can make a few bucks off them. There are a whole bunch of examples for this including Office for Mac, IE for Mac, Hyper-V support for Linux, OneNote, Bing, On{x} for Android.
Of course you can ask why is there no Office for Linux? I believe the answer is more simple than "EVIL M$'. It takes a huge amount of effort to port Office to a new platform(remember how Chrome faced a bunch of issues with Desktop Linux), and MS probably believes it is not cost effective since some of Linux' users want nothing to do with Office or any MS product anyway. If Linux breaks 10% of user share, maybe you'll see Office for it. There are very strong rumors that MS is going to launch Office for iPad at the same they're going to release Windows 8 and Windows RT tablets with Office, which is going to one less reason for users to buy a Windows RT tablet over the iPad.
Of course you can ask why is there no Office for Linux? I believe the answer is more simple than "EVIL M$'. It takes a huge amount of effort to port Office to a new platform(remember how Chrome faced a bunch of issues with Desktop Linux), and MS probably believes it is not cost effective since some of Linux' users want nothing to do with Office or any MS product anyway. If Linux breaks 10% of user share, maybe you'll see Office for it. There are very strong rumors that MS is going to launch Office for iPad at the same they're going to release Windows 8 and Windows RT tablets with Office, which is going to one less reason for users to buy a Windows RT tablet over the iPad.