In response to some push back, and at the risk of some repetition, I would like to clarify a bit more why I think there is no serious bar to the government revealing more about the legal basis for its action against al-Awlaki in Yemen.
There are very good reasons why the government might not want to officially and openly talk about a covert operation despite the fact that, as Ben says, everyone in the world knows about it. As Abe Shulsky and Gary Schmitt gingerly say of covert actions in their excellent (though somewhat outdated) book on intelligence:
[T]here may be cases in which a good deal of information about operations becomes public, but for diplomatic or other reasons, governments involved avoid officially acknowledging their connection with them. … [I]t is less provocative and less disruptive to diplomatic relations not to acknowledge an operation even if the country adversely affected by it is well aware of one’s involvement. The target country, either in the interests of good relations or because it cannot effectively prevent it, may ignore the covert action; it is much harder for it to do so if the government conducting it publicly acknowledges what it is doing.
I accept this and related explanations as reasons not to acknowledge the action officially. But maintaining technical covertness is not an absolute value or an absolute bar to more transparency. There are at least two competing factors to consider: