A priest only risks arrest when traveling through a realm with which he is at war. He cannot be arrested by anyone else. (Your own realm notwithstanding.) A unitless noble risks being arrested while travelling through enemy regions, at a rather high rate. That makes traveling unitless through regions wifth whom you are war a very dangerous ordeal. Priests also have the advantage of mid-turn travel, and multi-region hops. And they vare not arrested while traveling, or by routine patrols. That means an active player-lead police unit must be there to intercept them. (So hero characters, and anyone not leading a police unit can't arrest them.) And a priest can be saved from arrest by their followers. All these put together means that a priest, even one at war with the realm, a *much* less risky undertaking than a unitless noble. (Just ask Allison about her dash through Terran.)
