It was a requirement on government contract tenders. If you didn't support POSIX, you didn't get the contract and the government would have continued using SunOS or AIX or something instead of Windows NT. Remember that POSIX was designed to make these OSes somewhat interchangeable - they wouldn't order something they knew was incompatible with everything they already had.
The point still stands, that POSIX checkbox was relevant enough for spending the money in engineers salary during Windows NT 3.51 development.