Suggesting that all Ben-Hassrath are spies is like assuming that all craftsmen are carpenters. The Ben-Hassrath form a significant portion of the Qunari priesthood, tending to a variety of tasks within Qunari society.
In Qunari cities, Ben-Hassrath serve as something akin to a town guard, investigating anything that disrupts the orderly function of the city. Ben-Hassrath reeducators treat criminals and rebels against the Qun, determining whether they must destroy the subject's mind using a poison known as qamek. Those they can rehabilitate through treatment and education are later transferred to simple work details.
In contested or war-ridden areas, Ben-Hassrath coordinate with the Qunari military to track the dreaded Tal-Vashoth rebels, a function similar to bounty hunting. Outside Qunari borders, Ben-Hassrath agents primarily observe and report. While this may seem underhanded, it is no more than most nations do with their own spies, and the Qunari are (perhaps rightly, in retrospect) extremely concerned with the danger our culture's comparatively liberated mages pose.
Our assumption that all Ben-Hassrath are malicious spies bent on bringing Orlais into the Qun has no basis in fact. It is simple prejudice against a race whose appearance unsettles us, compounded by a guilty conscience at seeing how many peasants, especially elves, prefer life under the Qun to life under our empress. The Qunari are a society of people; people with a different culture and different values, but people nevertheless, as complex and nuanced as Orlais.
—An excerpt from The Lion and the Bull: Racial Bias and Oversimplification of Qunari Societal Roles in the Orlesian Court, author unknown, published by the University of Orlais