During the discussion of the commutation of Scooter Libby's sentence, one of the Republican talking points was that the pardon power (and implicitly, the power to commute sentences) is constitutionally committed to the President. That's right as far as it goes. Neither Congress nor the courts can reverse the President's decision to pardon someone (or, presumably to commute his sentence). See United States v. Klein. However, it doesn't follow that the President can pardon anyone he wants for any reason.
Suppose, for example, that the President decided to pardon all self-declared born-again Christians (but no one else) doing federal time for white-collar crimes. Surely that would violate the Establishment Clause. Likewise, a decision to pardon women (but only women) doing time for bank robbery would clearly violate the equal protection component of the Fifth Amendment's Due Process Clause.
There might be no remedy for these constitutional violations, short of impeachment, but then again there might be. In my first example, imagine that a non-born-again-Christian brought a habeas petition, alleging that his continued detention violates the Establishment Clause. If the judge agrees, Klein and the unreviewability of the pardon power would prevent him from ordering that the born-again criminals be re-imprisoned, but ordering the petitioner released would not interfere with the pardon power.
So, is it a constitutional violation to pardon high-ranking executive officials as a means of obstructing an investigation into potentially criminal wrongdoing in the executive branch? And if so, might Andy Borowitz have been inadvertently onto something when he jokingly suggested that thousands of people might be able to take advantage of Scooter's deal?
The answer to that second question is almost certainly "no," but NOT because, as Bush apologists claim, there was nothing constitutionally suspect about Bush's highly selective empathy for Libby. The fact that discretion is unreviewable does not render proper every exercise of that unreviewable discretion.