I think the actual problem is where to put him. No single state in the US volunteers to take him on. It's not so much about the rather small risk that he could be a terrorist (or might have become radicalized/motivated by treating him as such) but rather that some voters might THINK there is still a risk, however small. And a politician that advocates for taking him in will be (figuratively) flogged if the former prisoner commits anything worse than a parking ticket in his further life.

Similar pressures apply to Pakistan and to other countries who might feel even less responsibility to free him.