Of course, the President has the right to make that determination when it involves illegal combatants with whom we are at war. Why would he not; he is the Commander-in-Chief.
So far as Obama thinking we on this site are terrorists, Congress has not declared war on FreeRepublic. Not yet, anyway.
And I assure you that liberals will have no difficulty finding some kind of trumped-up legal reason to oppress whomever they choose. They'll do what they want, when they want, so long as the courts, the Congress and we let them.
Let's not confuse the legal exercise of power with the illegal abuse of power.
Congress has not declared war on anyone since 1941 but that never stopped the executive branch (now headed by the man you want to empower, Obama) from attacking individual liberty and shredding legal rights.