That driver is blacklisted in 8.1 and Microsoft demands from the vendor to pull it from their homepage.
Why they haven't revoked the signature, I don't know (maybe it would break the driver in Windows 8 too?). But the above is apparently what happened.
That was never my hypothesis! Quite the opposite. My hypothesis was that WindowsRT DOES support Ethernet, and Microsoft is blocking it artificially. Others in this thread tried to construct a technical issue for this (Dahat: "if you took a little time you might quickly realize which kind of drivers work and which don't.").