Stabbing some random person in the back doesn't require anything other than a sharp item. Just because someone is able to endanger others isn't a justifiable reason to lock them up. How much of a danger should a normal theodasian pose before they should be locked up without having done anything wrong?
Bringing up everyday Thedosians doesn't help, as the lives of peasants and elves suck. Keeping with ideals and moral standards I think helps the Libertarian arguments more. A peasant can be killed on a whim with little fuss, especially an elf. At least mages have more rights than a peasant, they can even organize into political parties.
Mages can kill multitudes by accident, have their soul trapped by a demon, cause a demon to trap mundane souls, mind control, and many other bad things a mundane can't. A mundane and mage live radically different lives, regardless of societal rules.