I've mentioned on another post that if you read the Dragon Age Core rulebook 2015 that was issued on line for people who wanted to play D&D style, there is a section on Ferelden to say that unlike other countries where the nobility seem to be sacrosanct, in Ferelden their role in society is to protect the freeholders. That is specifically seen as their job and if they fail in that role, then the freeholders feel perfectly entitled to give their allegiance and their tithes elsewhere.
In DAI the nobility of Ferelden were invisible. To my mind it was a fault of the writing to make them so but the fact is that apart from one or two individuals we encounter and some war table missions, the Ferelden nobility play no part in the events in the Ferelden side of the map. The Inquisition has to protect the ordinary people from mages, Templars, starvation, bandits, wolves, demons, Venatori and Red Templars. If we had not, they would have perished. So when a banner at the crossroads or in a village proclaims "under the protection of the Inquisition" it is hardly surprising that people are going to choose to pay their taxes to us as apparently Ferelden custom allows.
Caer Bronach was under control of bandits. That wasn't something that only a person with the mark could deal with, yet nothing had been done to shift them by the local nobility. In fact it would seem they took up occupation because that keep had been abandoned by whosoever was responsible for it. So, again, by Ferelden custom the Inquisition had simply assumed the role of the absent nobility. If Caer Bronach belonged to Teagan, why hadn't he done something about it? By the time we get to Crestwood, Redcliffe had already been restored to his family. If Caer Bronach belonged to some other noble, why hadn't they taken action? Unlike Haven, where we are confronted by an Orlesian noble who claimed title that had apparently been granted his wife by the monarch of Ferelden, no one ever turned up to take issue over our occupation of Caer Bronach until it was raised in Trespasser.
If any of these things had occurred, then Teagan's objections would have had more validity. If the monarch was happy giving title over Haven to an Orlesian, why couldn't they do the same for the Inquisition? As I say, I feel it was a shortcoming of the writing to have no one raising the slightest objection to our activities before the events of Trespasser. At the very least we should have had someone raising the matter before the council, perhaps Josephine drawing our attention to letters requesting the return of Caer Bronach to the crown and how did we want to respond? As it was, when Teagan says we should hand it back, according to the Core Rule book we were quite entitled to say, "By the laws of Ferelden, why should we? Let's put it to a vote of the freeholders."