Microsoft pays quite a few people to work on Code. I assume the code is open because they gain some value from community submitting PRs and is good publicity to develop it in the open on Github.

But, make no mistake, this is not a OSS project in the true sense of the word. MS will ALWAYS be the decision maker here and will do whatever it benefits them. They pay people to work on this full time, they get to have the last word. IT'S THAT SIMPLE.