They're terrified of Linux in alot of ways- its the only software stack enroaching on all of their markets simultaneously. Linux competition is so fierce that they have to fight not just for a companies as a whole- they have to undertake the impossible task of trying to force it out of the different departments of their own customers. Even the majority of Windows-only IT shops have a Linux box sitting around doing a task of one nature or another.
To stop this competition they have not only given away product, they've subsidized projects that would otherwise involve free software like Linux, and forced their biggest OEMs by offering heavy price reductions on their OS to manufacturers who sell their products exclusively. They have a team dedicated to 'outbreaks' of non-Microsoft products in the OS space, and have specific escalation guidelines for Free Software. At the point of these escalations, they're no longer selling software, they're forcing it on organizations by making deals impossible for other companies to make. This isn't a normal, fair, or sustainable business model, and it will eventually fail.