Newsweek revelations

It looks like a good, well-reported copy of Newsweek is about to drop on our doorstep. (Actually, two of them, since for some reason we seem to have two concurrent subscriptions. Sorry about all those trees.)
John Barry and an investigative team have been at work on one large chunk of the brutality in the gulag story. And Michael Isikoff has meanwhile gotten hold of both the Alberto Gonzales memo to W of Januray 25, 2002 urging him to junk the Geneva Conventions with respect to captives taken in Afghanistan, and an (unsuccessful) attempt by Colin Powell, also in memo form, to persuade Gonzo to change his mind.
These memos are now posted on the Newsweek/MSNBC website. Gonzo here and Powell here.
Gonzo was apparently most concerned to protect US intel operatives from any charges they might otherwise be subject to, in US courts, under the 1996 War Crimes Act, which bans any Americans from committing war crimes. (!)

To back up his argument that the Prez should issue a “determination” that Al-Qaeda and the Taliban did not qualify for any protections under the Geneva Conventions– a key provision of which forbids any physical or mental coercion of POWs as part of any interrogation– Gonzo wrote: “Your determination would create a reasonable basis in law that (the War Crimes Act) does not apply which would provide a solid defense to any future prosecution [of US interrogators using coercive methods].”
It’s not clear to me, under the US Constitution, why a simple Presidential “determination” of that nature would create the desired “basis in law”. Maybe someone could explain that to me?
For his part, Powell based his argument firmly on the dangers that such a determination might seriously undermine. Being as how he’d been an officer in the US ground forces for most of his professional life, he no doubt had a very lively appreciation of the value of the Geneva Conventions. H’mmm, wonder what happened to that?