. Meredith sends us after them because they have escaped from the Circle and stumped the Templars, two of them are blood Mages, and even abominations at that, the other was looking to score and he can be allowed to go free because he is harmless.
Yes, the elf who lived in the Alienage for years and even found a wife, but lost his mind after ten years in the Circle of Kirkwall, and the human who became an abomination after having spent years in the Circle of Ferelden without succumbing, and even surviving the Blight. Those examples certainly addressed that the Gallows isn't a pleasant place.
These Mages came from the Circle and were proven blood Mages, these are not the only ones, others are mentioned in dialogue
So an entire population should be executed because there was one insane blood mage, one abomination, and some other mages mentioned in dialogue? I'm not seeing that as sufficient grounds to kill an entire population.
In fact, it's a moot point, as the only reason the Right of Annulment is called is because Meredith wants to kill all the mages to appease a hypothetical mob who will want them all dead simply for being mages.