The Jack/Juliet thing finished when Juliet was sewing Jack up after the appendectomy. You remember Juliet said some things to Kate about Jack kissing her; but she knew he was only trying to prove he didn't love someone else (Kate). She also knew Jack could hear what she was saying to Kate because he had come out of the anaesthesia.
So he didn't have to protect Juliet from Ben cos their little 'thing' ended just before he left the island.
Presumably Ben thought Juliet would leave the island too or he was too busy grieving for his daughter and fighting Keamy to worry about his: "Juliet is my woman" fixation!
