Seanchan are not Darkfriends. They are empire in the height of power and expanding. hence they are unstoppable. Rebellion is unthinkable in times when there's so much to loot. Still there's evidence that knowledge of the nature of the a'dam is going to undermine their might. But Mat is ta'veren, isn't he? He should find the way to lead them. Or the way should find him. They have nowhere to move now without strikng at the power. The choice is not rich: Rand with his armies, Prophet or Graendal. They can possibly beat Dragonsworn, that means they are going there to meet Perrin.