First of all, why would the Mac be a competitor platform? MS always has supported the Mac with their software. Then, it could be strategic. Consider the scenar

First of all, why would the Mac be a competitor platform? MS always has supported the Mac with their software. Then, it could be strategic. Consider the scenario in which the ARM based chips maintain a steady advantage vs. the x86-based chips. Microsoft then certainly wants to have a share of that market on Windows and not leave it exclusively to Mac OS. Considering how that depends on the availabilty of native ARM in software, they probably want to run on the one nice ARM implementation available on the desktop right now.