That voice man...That voice can get chicks and inspire men.
Actually though there were books saying the Telvanni were a major part of the slave trade there were few slaves actually in-game that belonged to the Telvanni, and even so most slaves are weak and would be poor specimens to anyone but a novice and there were Telvanni like Fyr who expanded his life span by 4000 years and used Necromancy to create instead of enslave. Thus further proving my point and while the University did have restrictions the body of Criminals and traitors is perfectly fine since most of the time they were murderers, thus a fitting punishment. Also not all Necromancer aspired to become liches, Fyr being my example again as well as several other Telvanni necromancers who lived to an old age due to necromancy and never went towards lichdom and finally the whole soul thing is more of a theory I had

Edit- also where did it say necromancy was the cause of extending their lives? I do believe that is very likely, but do not recall them ever saying it. Fyr could have used corpus for all I know.