Although I'd wish people would take a bit more of a consideration when understanding the scope of the problem with software patents.
For this example... The drivers are, I am guessing, are I/O drivers that implement some paravirtualization features to increase the performance and compatibility of running Linux in Hyper-V.
Well those patents that Microsoft may possibly have that covers some of the Hyper-V paravirt features may apply to the Linux driver code. But it's pretty likely that any patents would apply to paravirt techniques used in the Linux-KVM stuff or in Xen stuff.
So getting Microsoft to contribute code to the kernel means that not only are they probably sacrificing much of their ability to go after paravirt features used in KVM or Xen, it is quite likely a gain for Linux kernel in other ways.