They're not. After you enable the Linux subsystem you get the userland from Canonical under Ubuntu's license, Microsoft ships WSL and CLI shims triggering the download of the Ubuntu userland.
They just don't see it as worth the energy for them to use it.
I can't disagree with that strategy (if no one is really pushing you to do X, why bother), at Google we do the same with android mostly, but for different reasons:
We don't use GPLv3 because the OEMs/etc don't want it, and rip it out anyway :)
Similarly, i expect in Apple's case, they don't use it in parts they may keep common to ios/macos because then they can't keep them common.
In any case, the TL;DR is that there aren't open legal questions or magic things they are waiting to play out. They've made their choice and have been sticking by it.
I'm curious why you think it opens up legal questions, etc. They've pretty definitively stated they just don't want to use it.
There are no vague or mysterious questions they are waiting to get answers to or anything like that.