I think the idea is that right now, MS doesn't have much of a choice. Like it or not (and I doubt MS likes it), the server world runs on UNIX/Linux, and out of the four "mainstream" OSs (Windows/Mac /GNU+Linux/Chromebooks+Linux), it's the only one without a UNIXlike back, so while MS is fine on the desktop, they'll have long term problems if developers move to Mac or Linux and the community will see alternatives to Windows.

But in the end, it's not as if Windows is Open Source, and there's no guarantee that WSL will exist in it's current shape forever.

This is actually the whole EEE strategy. You embrace it, as a monopoly you can push your copy over any other, then you "extend" and improve it (so there'll be code which will only work on WSL) and then ... You won.