As Proweler pointed out above, there is a lot of work that goes into the preparation of a corpse for necromantic sevantile usage.
Nonetheless, there doesn't seem to be a lot of work that goes into summoning skeletons, zombies, and/or other forms of undead through spells that, although the creature summoned has necromantic ties, do not seem to be necromantic spells due to being taught by the Mages' Guild post-ban.
Why?
My theory, which is unsupported through text at this time, is that the spells that summon these undead creatures are actually summoning them from Quagmire, the realm of the Daedric Prince of Nightmares, Vaernima.
I state this for two reasons: (1) undead creatures are terrifying to mortals, thus their connection to the fear that Vaernima thrives upon, and (2) the offering that Vaernima needed at her shrine for the Daedric Quest in TES IV is a black soul gem, which clearly has necromantic ties.
So while the summon [undead] spells might summon necromantic subjects, it is not necromancy that pulls them from Oblivion. In the same sense, while necromancers may be able to summon those zombies and such, they are using Conjuration. The shambling zombies, skeletons and "permanent" undead creatures were created with necromancy.
Nonetheless, there doesn't seem to be a lot of work that goes into summoning skeletons, zombies, and/or other forms of undead through spells that, although the creature summoned has necromantic ties, do not seem to be necromantic spells due to being taught by the Mages' Guild post-ban.
Why?
My theory, which is unsupported through text at this time, is that the spells that summon these undead creatures are actually summoning them from Quagmire, the realm of the Daedric Prince of Nightmares, Vaernima.
I state this for two reasons: (1) undead creatures are terrifying to mortals, thus their connection to the fear that Vaernima thrives upon, and (2) the offering that Vaernima needed at her shrine for the Daedric Quest in TES IV is a black soul gem, which clearly has necromantic ties.
So while the summon [undead] spells might summon necromantic subjects, it is not necromancy that pulls them from Oblivion. In the same sense, while necromancers may be able to summon those zombies and such, they are using Conjuration. The shambling zombies, skeletons and "permanent" undead creatures were created with necromancy.
I find the theory highly logical as a workaround explanation for game mechanics.