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.

It's a controlled exception. Much like the League restricts the use of any potions sans what is 'sold' at the shop, they are making an exception for Mundo's anabolics and Singed's poisons, because those are their core abilities.

Okay, there is really only one problem that I have with this theory. We don't fully understand what "dying" on the field of justice actually means. For all intents and purposes it seems to be an infinite loop where your body and the leagues version of a soul never really die or move on... you just seem to pull a Red and white out when you are defeated and then pop back up some where. So as much as this warrants possible discussion,I'm not sure if it can go anywhere unless we have a full defintive answer on what happens when a champion dies on a field of justice under the call of a summoner. I'd honestly think you'd have a stronger case arguing for Necromancy in Yorick's standard abilites, he technically summons spirits to do his bidding. Although just like with Morde's ultimate I officially have no bloody idea as to what happens or if there is even a spirit avaliable to be manipulated... RIOT! PLS!!!

From http://na.leagueoflegends.com/story/...20-june-21-cle
The first paragraph for The Mailbag of Justice