So Microsoft is not a Linux contributor but they have merely contributed code to the Linux kernel.
And enough to appear as one of the 20 biggest contributors.
Please, forget about this "oh, they MUST make Linux better on its own" mentality. As you said, Microsoft contributed to make it play nicer to their software... So what? Put yourself in their shoes: wouldn't you want your own software to be compatible?
Hell, if they contributed to fix the software controller in their espresso machines, it would still be valid.