Well mages really aren't slaves. Chantry can't tell mages what to do, they just hold them locked up, but inside they are free to do whatever they want. ( if it doesn't brake the law ). So that sounds like prisoners to me.
Well, merriam-webster defines 'prisoner' as "a person deprived of liberty and kept under involuntary restraint, confinement, or custody."
Which is pretty much indisputable and completely correct.
Whereas the most relevant definition of 'slave' is "someone who is legally owned by another person and is forced to work for that person without pay."
Which is indisputably not the case by objective facts: mages aren't owned, aren't forced to work, and when they do work it isn't without pay.
Slave and prisoner are not synonyms, and the situation of mages in the Towers is far closer to the former than the later.