For an excellent analysis of the unitary executive rule of Art. II, see Jack Goldsmith, Maximum Executive Power and the Fate of the Unitary Executive: Link
Here is a money quote:
That conception has four related tenets: (a) the Constitution vests all of the Executive power in the president; (b) all subordinate executive branch officials are removable at will by the president; (c) the president’s Article II duty to “take Care that the Laws be faithfully executed” entails an exclusive presidential power to decide which laws to enforce or not to enforce; and (d) the president can thus direct and control all subordinate executive officials.
