Except that Necromancy itself is illegal in Cyrodiil, as evidenced by a conversation with a certain Alchemist in Skingrad.
Bull honky. The imperial government regularly donates corpses of bandits to necromancers. Serves two purposes for the empire: disposes bodies of people they don't want to deal with, and can serve as a deterrant (you see that walking abomination there, bandits? Yeah, that's what you'll end up if you pull that crap). Plus, IT'S ONLY ILLEGAL IN THE MAGES GUILD! Outside the guild, they can do what they want, except for breaking county and imperial laws, WHICH DOESN'T PLACE NECROMANCY AS BEING ILLEGAL.
Even without the act itself, you have to figure that (almost) every Necromancer is doing something illegal. Where do they get their corpses? Either they steal them from graves (which, as shown in Thoronir's quest, is punishable by death), they kill someone (obviously illegal), or they somehow stumble across enough corpses of unfortunate adventurers to constitute meaningful research or the practice of the magic itself.
I also love making gross generalizations of things I don't fully understand!
Like I said earlier, bodies of bandits are donated to necromancers at a pretty sustainable pace. But hey, if you want more, they can travel to a near-by cave and dispose of some bandits. Or, they could, you know, pay people legally for bodies. Grandpa is sick, can't afford to bury him? Necromancer in search of corpses, willing to pay? You got yourself a win-win. Family doesn't have to bury grandpa, and the necromancers gets his body. And hey, what's wrong with cleaning up caves of dead adventurers? If they have been lying around there for some time, obviously no one give a rats butt about them. Necromancers make great clean-up crews!
Grave stealing is obviously illegal, and should remain as such, along with murder.