I'm assuming if Microsoft could legally stop this, they would.
Likely the interfaces aren't copyrightable and this is probably a clean implementation -- but I'm sure if Microsoft could trot out a patent or something else to stop people they would.
I can't imagine they want implementations of their stuff out there. (Granted, they mostly started out by implementing other people's stuff, so there may not be much they can do about it.)
Well if this article is still valid, then I would say they don't mind Samba. http://linux.slashdot.org/story/08/10/23/1441200/microsoft-working-for-samba-interoperability