I definitely think Kate has chosen Sawyer. (big time) However, she knows (or believes) that she has no where to run.....plus Jack did not direct them to set Sawyer free .....ONLY Kate. I am pretty sure that is out of spite. I think Kate has sensed Jack's jealousy (especially when she visited him in his cell earlier) in this regard and so needs to play it cool...knowing that Sawyers survival could rest in Jacks hands. She saw how Jack reacted when she pleaded for Sawyers life earlier, he totally shut her down. Her leaving right now is not an appealing prospect to her because she would be leaving both of them behind to try to get off of one island and back to another.