Obvious question for the historians: did any C13th authors write for profit in a way that we would recognise? I doubt it, and it seems like a ludicrous idea in a world where everything had to be copied by hand, and most authors were churchmen with supported lives. So the idea that magi (who also have supported lives) would do it seems grossly anachronistic. You write because you have something important to say, not for the vis.
But also: Cow and Calf isn't copyright, and viewing it that way is another anachronism.