Well, because the strategy was really nasty stuff and acquiring github might fit into this kind of pattern.
But don't let actual evidence stop your pitchforks.
However, I am keeping my skepticism when they embrace open-source and am curious which 'actual evidence' you are referring to?
So far, many open-source efforts look more like marketing to me. For instance, WSL makes sense to keep people in a Microsoft environment (and to me at seems that more and more frameworks favor Linux and Mac over Windows).