While I'm not sure I agree with Mr. Posner's normative assertion that "Obama should treasure this gift," I agree that the presidency seldom voluntarily cedes power. It might lose power (e.g., the War Powers Act, Truman at Youngstown, Mr. Clinton's impeachment), but it won't, usually, cede power. Even Thomas Jefferson, who had inveighed against the Alien and Sedition Acts, let them expire instead of asking the Congress to explicitly repeal them.
Veterans of the Clinton era know full well that unexpected circumstances require executive action where existing statutory authority is inadequate and adequate statutory authority is unforthcoming. Why tie Obama’s hands by repudiating the flexible standards that Bush lawyers have labored to enlarge? Obama should treasure this gift from the Bush administration rather than return it: it will come in handy when Republicans complain of executive overreaching over the next 4-8 years.
The new president will exercise a lot of power, but this exercise of power will take place within institutional constraints, pathways and incentives that will make his decisions resemble Mr. Bush's by and by.