If so, then how is he able to cast it within the bounds of a Field of Justice?
According to the first JoJ, the League allows only two city-states to study necromancy. While they haven't banned its use outside of the League (hence why it is okay for Yorick to perform his duties), "The League does not sanction the use of necromancy, and the study of necromancy and the manufacture of necromantic items are not allowed within the Arcanum Majoris." (JoJ Issue 1, Piracy Strikes Demacian Navy section)
With that said, Yorick's ultimate creates a revenant so that the death of the champion will result in a recalling of the spirit. My question then is, if this in fact necromancy (pretty sure summoning the soul of a recently departed into a visible ghost is necromancy) why can he use it? As much as the League cautions against its use and restricts it within the Arcanum Majoris, are they going to let a champion freely use it in a Field of Justice? Or are the supposed powers of the nexuses and summoners in the League powerful enough that this use of necromancy isn't seen as an issue?
Perhaps I am misreading something, but I am pretty sure this is a little inconsistent.