They were powerless against it.
Ferelden isn't Tevinter. Tevinter is built on blood magic. Ferelden isn't.
Ferelden would be more susceptible to mind control because it's not as commonplace as it is in Tevinter. Feynriel himself notes that he saw his own tutor kill a rival Magister in battle.
I think -- key word being think, as I don't know -- that the Magisters might view mind control over other Magisters as being crude and even a sign that they're not that strong to take on a rival Magister in battle themselves, so that's why they do the duels.