I believe Cooper's claim is that he heard it from Rove - he mentioned it to Libby because he heard about it from Rove. None of the reporters claim prior knowledge.
yes, I know the case is not about when the reporters knew. but had it been established that they knew already, it simply lends credence to the idea that the only reason they were asking - was to entrap Rove and Libby. already this case involves no crime on the underlying statute, if we then add entrapment to that - making the indictments seem "fair" becomes even harder.
If Libby thought the investigation was bogus, then he should have seaid so. The indictment recites a story that paints Libby as having lied to investigators.
So, is your argument that it is okay to lie to investigators? I'm not sure there is entrapment - not on a read of the indictment. Libby learned about Plame, the investigators asked him some questions about where he learned about Plame, and he didn't tell them the truth (so reads the allegations of the indictment). He could have just as easily said, " I called the CIA and they told me, then I spread the rumor around to a buch of dumbass reporters to see how fast the rumor would spread." No crime there.