I would go along with this more than anything. Ul'dah is having tons of problems with refugees and rebuilding; taking in tempered prisoners and waiting/hoping for the process to be reversed (I do wonder if its possible to reverse it 100%; perhaps it weakens to the point of the tempered to regain some free will, but they have that bit of their Primal still in them, and it can be "rekindled" easily or something) would probably do more harm than good.
Having the general population think tempering is non-reversible is likely as much a tool of control as anything; keeps merchants and the like from dealing with beastmen, adventurers too, and the general population fearful of the beastmen.
The issue, then, would be with the Scions. Where are they getting the info that tempering is permanent? If they're just taking what someone said (like, whoever convinced everyone else) and not checking...or did they look into it themselves? I'm more curious about that...
Also, what is the stance on tempering's permanence from the other two city-states? We really only get the view from Ul'dah (I think, I can't remember).
Its possible, but we can't be 100% sure. It may be that certain people need to be "re-tempered" often (every few months or so), and some get tempered almost permanently (years between tempering). If tempered prisoners are trying to carry out the will of their primal while waiting for the tempering to wear off...it could be very hard to justify keeping them alive long enough for it to wear off.
