And the TDS specification is also available (no registration needed to read the docs). So anyone can write their own freely licensed driver (much like FreeTDS). I've been thinking of writing a native Haskell port, myself, as SQL Server on Linux is so much easier to start with (SQL Server is one of the systems that has almost 0 support anywhere in our ecosystem, but I know of more than one person who'd like to see it).
So while SQL server is unavailable, free client drivers will hopefully begin popping up more and more and will become accessible through e.g. Debian repositories, with time.
I looked at the TDS spec and it's one of the most confusingly written protocol specifications I've seen. The unusual/somewhat verbose wording, general lack of examples (except for a brief section at the end, in hexdump and XML(!?)) and overall (mis)organisation give the impression of hastily redacted internal documentation and/or deliberate attempts to discourage implementation while still being able to claim the protocol is "open".