In this case, the issue revolved around the treatment of a United States citizen, not a foreign national as was the case in Hamdan. In Hamdi, the Supreme Court held that it was not legal to hold an American citizen indefinitely as an illegal enemy combatant unless the citizen had been determined to be an illegal enemy combatant by an impartial decision-maker.
Because there was no single opinion that was able to gain a majority, the reasoning behind this ruling is unclear. Different justices would have put different levels of restrictions on the executive's right to hold American citizens. The only clear holding in the case is that it is at least necessary to have an impartial arbiter declare an American citizen to be an illegal enemy combatant before he or she can be held indefinitely.