Apply some common sense, you think they let MS employees just commit to any codebase they like? And this is leaving aside all the annoying steps of locating their repository, figuring out how to build it, testing, finding out the right person to send the patch to, going through code review.
"you think they let MS employees just commit to any codebase they like?"
That's pretty much how it works at Google. The code owners have to review and approve your change (and can reject it), but it's common (even encouraged) to commit a bugfix or new feature to someone else's project.
How is that any different than a central maintainer of Linux accepting patches and reviewing them before they go in?
The only difference is that you can see the source and test on your own machine first.
MS employees don't have to commit to any codebase they like, but they should be able to obtain the actual source, so that "with enough eyes, all bugs are shallow".
Actually...I heard Google has developed a lot of infrastructure to allow for this: one repository, you can submit changes that are then automatically routed for review and test and such.
It's one of the things Nadella has talked about, and plans to address over time (the extreme compartmentalism of and fierce "not made here" attitude a lot of departments/groups within Microsoft have developed over the past few decades.
There are ways to get rid of NIH without losing code quality, testings, responsibility, etc. Nadella in particular is talking about encouraging collaboration between departments.
PS: While the linked article is very relevant here, I'm getting tired of people always linking to Joel's blog -- he's not the definitive software spokesperson; has strong opinions which have been shaped by years at Microsoft - and a lot of his opinions are boxed into those views.
If you pay any attention to Microsoft, their business practices, news, reports, blogs, etc... then you would know the parent comment is pretty much spot on.