well, given how much power she had, I believe the king's guard would do anything for her. whether it be from loyalty or from fear of what she would do to them if she becomes free and knowing they did nothing while she suffered in prison.
the only thing that I can see that makes the situation believable to me is that her uncle is in king's landing and acting as hand of the king. I am assuming since he has not freed her or gone to visit her that he simply doesn't give a damn and probably feels it's better she stays in prison and out of the way.