I think what the problem is, is that with Jodi Arias and Casey Anthony , no matter what the feelings about them, there is acknowledgment that a crime was committed, and hence the prosecutor has the right to apply the rack, to get them to give witness. With Knox, there is a vast army of people who feel she ought never have been arrested; ergo the prosecutor ought not be holding her feet to the fire, in the first place.
And if she hadn't lied she may not have been arrested. I don't think Casey was arrested until her lies started coming out, was she?
Amanda was arrested after Raffeale told the police Amanda told him to lie for her. At least that's my understanding.