OR - I had an epiphany (ouch!*) - since Juliet and Ben were once involved, even possibly married, maybe Ben gets rid of anyone she's interested in - Ethan (maybe), and Goodwin (I'm pretty sure), were sent off to "live" among the Losties - maybe he's tired of it all (or still bitter because now she's sweet on Jack) and ended up finally just banishing Juliet. But I think she is still spying, possibly for those in the Other's camp "who want a change".

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`Dear, dear! How queer everything is to-day! And yesterday things went on just as usual. I wonder if I've been changed in the night? Let me think: was I the same when I got up this morning? I almost think I can remember feeling a little different. But if I'm not the same, the next question is, Who in the world am I? Ah, that's the great puzzle!'

-- Alice, from “Alice’s Adventure’s in Wonderland”