The did no evil and their fear of punishment does not prove guilt, it merely proves they fear punishment, rightfully or wrongly. Do people pay protection money to the mafia, because they acknowledge the right of the don to a portion of their earnings? No, because they fear that, justly or unjustly, they will suffer worse from a refusal to pay. Myrelle said what they did was legal, therefore she truly believed it. That does not change the fact that the Hall might decide to butt in and punish them unjustly. It should be the Amyrlin's duty to protect her daughters from that kind of injustice, but left caught between two wolves, they chose the one that promised less pain. These are women who think spankings are something to fear. We are not talking about people with great amounts of personal fortitude. The whole hierarchy system of the Aes Sedai forces them to submit to the wishes of superiors. This oath simply amounts to one more person with intrusive authority over them. After decades of Tower conditioning, they simply don't see it as all that big of a deal, and take the Oath rather than lose political face.