For whatever the resaon might be, Microsoft also does this :
http://www.itproportal.com/2011/07/18/microsoft-contribution...
"In July 2009 Microsoft submitted Hyper-V drivers to the kernel, which improve the performance of virtual Linux guest systems in a Windows hosted environment. Microsoft was forced to submit the code when it was discovered that Microsoft had incorporated a Hyper-V network driver with GPL-licensed components statically linked to closed-source binaries."
So obviously Microsoft didn't do it for altruistic motives.
"whatever the reason might be" as if it wasn't clearly stated in the article.