From a quick search, it seems this "tax" is part of an ancient (roman law) land contract called Emphyteusis[1] that's still recognized as a valid by the italian

From a quick search, it seems this "tax" is part of an ancient (roman law) land contract called Emphyteusis[1] that's still recognized as a valid by the italian civil codex, despite not being in common use. I don't think it has anything to do with special nobiliar priviledges: it's just that these barons claim to own land and the relative rights under this contract. Apparently, the land owner has no obligations, but the lease-holder instead is bound to pay an annual canon and "improve the land", whatever that means, perpetually. [1]: https://it.wikipedia.org/wiki/Enfiteusi https://it.wikipedia.org/wiki/Enfiteusi