Posix compliance was/is often a checkbox requirement to bid for government contracts. So M$ tossed in the Posix subsystem so they didn't get excluded based on

Posix compliance was/is often a checkbox requirement to bid for government contracts. So M$ tossed in the Posix subsystem so they didn't get excluded based on that requirement. Lots of otherwise odd features in software fall into this category (See also: Apple A/UX).