It depends on how corrupt the Magisters are, but if I see how openly the Flencer butchers in the big hall, I'm pretty sure the magisters on the island are deeply corrupted. Their intention is not to imprison sourcerers for the good of the world, but to use them as their guinea pigs and mindless soldiers (monks). The same reason why they want Cork to see the real truth about the Magister order. Only a few perhaps like the blinded one could probably be innocent enough and are perhaps fresh recruits. Dallis herself is pretty obvious a sourcerer, in the first scene she already rips the source out of somebody and Shriekers seems to be products of source experiments as.
