The advantage the Masons have is that they know who two other people are that are also Masons - and therefore neither the Seer nor a Werewolf. You can't reveal directly or indirectly that you (or one of the others) is likely or definitely a mason. So the Masons know they don't want the other Masons to die - but they can't let other people know that unless they pretend they have other reasons or arguments.