Juliet let Jack off the hook because she knew Jack really loved Kate. Me being a woman I can tell you that most women would let a man off the hook rather than have a man with you who would sooner be with someone else.
Jack only got close to Juliet because I think he knew she liked him (it soothed his damaged ego) and he thought that Kate was going to be an item with Sawyer.
You have to look at this situation from a woman's viewpoint. Plus Juliet has been really heroic letting Kate know what the score is re Jack's feelings towards her.
Juliet has been a good and cool woman.
