During his entire time on the show almost everything Ben has said can be shownn to have been a lie. In fact, I can think of only two things Ben has said that were true. The first when he said at Locke's burial that John Locke was a better man than he would ever be (or words to that effect) and when he said to Juliet, "Guess I'm out of the book club, huh?" Other than that, lies. So why do any of them believe him and return to the island? Other than for the convenience of the story?