Officials decided that it would be imprudent to reverse course so abruptly because they realized they didn't yet have a full picture of the intelligence methods and secrets that underlay the privilege's assertions, because the privilege might correctly protect a state secret, and because the domino effect of retracting it could harm legitimate cases, both civil and criminal, that are already in progress.
"If you decide today precipitously to waive this privilege, you can't get it back,an administration official said. "If you decide to assert it, you can always retract it in the future."
I don't agree with Greenwald that Ambinder's post is nothing more than "subservient pro-administration stenography" because Ambinder cites an anonymous source in the administration and doesn't question the source's reasoning; before Ambinder's post we didn't know the rationale behind the administration's decision to adopt the Bush administration's position as their own, and now thanks to Ambinder we do. Or at least we know what they're willing to say publicly (if anonymously.)
What I do agree with Greenwald about, is the absurdity of the idea that the Obama administration would be "precipitously" waiving the privilege. That's ridiculous on its face. Senior members of the Obama DOJ have studied, and criticized, the assertion of the state secrets privilege by the Bush administration for years now. To argue that coming into office they were not prepared for this case is disingenuous at best.
Now I'm willing to give the Obama administration a the benefit of the doubt, but only to a slight extent. The assertion of this privilege to deny plaintiffs even their day in court to air their grievances is wrong, and is nothing more than a means to defend government wrong-doing and lawbreaking. If members of the Obama administration do not make it clear that they agree, sooner rather than later, than we are entirely justified to lambaste the Obama administration for their failure to commit to the ideals that Obama himself articulated on the campaign trail.