So if people won't pay for operating systems what will they pay for? Stuff that still costs them money, i.e. hardware infrastructure and attendant maintenance and administration. Hello cloud. Development shops are happily sending Amazon millions of dollars a month so they don't have to buy servers, set up server rooms, hire system administrators, and worry about things like air conditioning, big UPS systems and emergency power generators, etc.
Alongside Amazon, Microsoft is doing pretty well with Azure. And you can run free operating systems on Azure, and .NET is already well supported on Azure. So what will sell more Azure? Make .NET run on the free operating systems. Now all the developers who like the free OSs on their personal dev systems are suddenly able to develop stuff they can deploy on Azure. What about all the devs that like Macs? No problem, make .NET run on Mac OS X as well.
I think this is all about making Azure services more compelling and more able to compete with Amazon than it is wanting to do "nice things" for the open source community.