They are fighting with VMWare in the virtualization market. They need this to have a better support of HyperV in Linux. That means work with the kernel developers and their rules.
I wouldn't touch any code contributed by microsoft with a 10 foot pole. There is bound to be some catch or strategic trick here.
Also, they don't need to change Windows, just the hypervisor. Though it would probably break other OSs too, not just Linux.