They test that and it doesn't work. The oath won't allow them to say something else that people will hear as a lie.

Rand returns to Bandar Eben and restores the supplies there so that the people of the city can be fed. Darkfriends can also be blinded by the halo of light. He identifies the dark friends among the High Lords and Ladies of Tear by lining them up and forcing each of them to look into his eyes. Weiramon and Anaiyella can't do it, so he calls them out and tells them to leave and tell all of the other Dark Friends that they can no longer hide among his allies. When he visits Egwene in the White Tower, even though he's shielded by a circle of 13, Egwene and the Aes Sedai realize that they're powerless to hold him if he wants to leave. Rand says at one point that he no longer fears the shadow because "The time when it could silence me quietly-and therefore win- has passed. The confrontation is assured and the scream that begins the avalanche has been sounded." There's orders of magnitude in power between Rand and everyone else at this point (with the exception of Moridin).