^^Article II, Section 2 of that peskie little Constitution of the United States of America. And even then, the recess appointment must be approved by the Senate before the end of session or the position becomes vacant again.The President shall have power to fill up all Vacancies that may happen during the Recess of the Senate, by granting Commissions which shall expire at the End of their next Session.
That's why Presidents must give-and-take on some issues. Obama's not done much giving. Karma's a bitch.