NOT a political prerogative?

Its worth noting that an alternative view is that Tsarnaev cannot be detained as an enemy combatant because he is not a member of Al`Q and therefore the Congression "Authorization for the Use of Military Force" does not apply to him. Were he an actual member of Al'Q, the Obama Admin might have handled this differently:

"I am not aware of any evidence so far that the Boston suspect is part of any organized group, let alone al-Qaida, the Taliban, or one of their affiliates–the only organizations whose members are subject to detention under the Authorization for Use of Military Force, as it has been consistently interpreted by all three branches of our government," Levin noted. "In the absence of such evidence I know of no legal basis for his detention as an enemy combatant."