[0] https://stuff.mit.edu/afs/sipb/project/android/docs/distribu...
I'm unsure when the branding decision was made, but the possibility of again involving Android support was probably considered.
https://en.m.wikipedia.org/wiki/Windows_Services_for_UNIX
Funnily enough it lacks "subsystem" but its ancestor was in 1993 "Microsoft POSIX subsystem".
But it's just like Microsoft to blindly settle on a name that doesn't make sense, based on advice from legal, and not probe it a bit more, or play with it to make more sense while still fitting the guidelines of legal advice, and to cite the legal guidance as the reason it has to be mediocre and cannot change. (Source: I once worked for MS.)
Edit: to be clear, I'm also reacting to the "windows subsystem for Linux", which they used the same justification for.
> Descriptive Fair Use: Found at 15 U.S.C. § 1115(b)(4), the descriptive fair use defense protects your ability to use ordinary words to describe your own goods or services, even if those words happen to be part of someone's trademark.
https://www.dmlp.org/legal-guide/using-trademarks-others
Since the Windows Subsystem for Android actually runs Android, "Windows Subsystem for Android" is a product name and not just a description, and "Android" isn't a generic descriptive term in this context, it seems to me that Microsoft is using Google's Android trademark in a way that isn't protected as fair use.
Really I wasn't trying to name the exact defence but gesture at the principal that trade marks prevent competitors from using names that might confuse customers, but they don't prevent competitors (or anyone else) from talking about the product. It was just a coincidence that I used a word that happened to also be the name of a specific kind of fair use.