Its just too obvious a way to allow someone to betray the black ajah. There have to be other ways to achieve the effect of both oaths without also allowing such a dumb way for someone to betray the secrets of the black ajah. I don't know if RJ came up with that language or if he never decided how Verin would be able to betray the black ajah and Sanderson had to come up with that himself. Either way, it feels like bad writing to me.