I recently came to the conclusion that Szeth's oathstone is not just a rock. It works similarly to a soulcaster, which we know from Navani's Notebook, works by trapping truthspren inside of it- that gives the user the power to soulcast without having a personal bond with a truthspren like Shallan does. I think an honorspren is encased inside of Szeth's oathstone- that would give him the ability to surgebind for he would have an indirect bond with said honorspren- so Szeth would lose his powers if he disobeyed his masters, but he does have a choice he can break the bond. It is like when Syl offered Kaladin the choice to end their bond but he would lose surgebinding, and she would lose her memory.
Please tell me what you think of this theory.