I'm 10,000% sure that Microsoft knows that this is the general perception and has done their research. Maybe there's more of a market than you think.
Government and corporate users of Linux. There are a lot of Excel horrors that OpenOffice chokes on out there, and it's not always possible to convince other organizations to keep their horrors to themselves. Buying Office for Linux would be less bad than buying Windows to install on a VM to run Office on.
This is similar to what I do. I use a Mac only because I am expected to do things with Office (OpenOffice/LibreOffice just doesn't cut it) and Outlook is great. Though, I do all of my development/research on a Linux VM or Linux cluster. I only have OSX because of Office. I would drop OSX in a heartbeat if I could have Office on Linux. Though, it has to have a flawless Outlook, too.