We were given an odd piece of information about Juliet in "Not In Portland". She tells Jack that she has been on the Island for "3 years, 2 months, and 28 days", which according to the math dates her arrival at 9/5/2001. Today I was thinking that maybe the true reason that Ben can't let her go back to The World, even if he wanted to, is because an elaborate lie has possibly been publicly fabricated about her death, along the lines of the one we are led to believe has been set up about the 815 passengers' deaths.
Think about why Mittelos may have chosen her over other talented and dedicated researchers. Sure, she is said to be a fertility genius, but that was about all that she had in her life really. Besides helping her sick sister conceive, artificially or otherwise, she had no life outside of her work. And her only other living tie to The World that we know of, was smacked by a bus and killed. BTW, if Desmond's ep was called "Flashes Before Your Eyes", Juliet's ep should have been called "Smashes Before Your Eyes"! That was a very unnerving scene.
Therefore if Juliet was chosen because she has virtually no links to The World (other than Rachel who They will use to manipulate Juliet emotionally if need be), Mittelos could have staged her death in the public news as well. Consider that they may have assumed that, in spite of her genius, Juliet was a complete loser emotionally. They could have pegged her for being the perfect stooge who would gulp down the "kool-aid", chuck the world and all it doesn't have to offer her, and who would be willing to live an "idyllic" life in Othertown doing the research that she loved for the rest of her life. They would watch over Rachel and her child (another post entirely!), and Juliet in her gratefulness would stay to work on the Island's great conundrum forever.
Then along came the attack on 9/11 right after she left..how convenient for Mittelos. They don't have to stage a car wreck to "kill" her, they can merely say that she happened to be at a Mittelos meeting in the WTC that morning. No remains, end of discussion. But something happened to Juliet on the island that they didn't expect...she seems to have grown a backbone. The little mouse became a lioness. Her frustration in the failure to solve the fertility problem, her small town blues, and her love for her sister, gave her a fierce determination to get the heck off of Craphole Island. Bad news for Ben, as now he's stuck in a scenario that they didn't make a contingency plan for -- Juliet's rebellion. Maybe Ben actually would like to get rid of her and the trouble that she causes but he can't, because she's supposed to be dead now which could not be explained if she returned to The World. So now Juliet dreams of being Carrie and telekinetically killing Ben, or getting someone else to kill him. In the end, like with Walt, I think that the Others got more than they bargained for with Juliet, even after all of their best laid plans.