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.
the only reason the questions were asked by the investigators in the first place, is because reporters who claim to be totally ignorant are saying "hey, we heard everything from Rove and Libby". while it may not be OK to lie to the investigators, if the only reason they were steered your way to begin with is because you were setup by the reporters, and they can be shown to have lied, that means alot in my book.
the case could well have gone the other way - suppose after hearing from Cooper/Miller/Russert that they had no prior knowledge of who Wilson's wife was, that the FBI investigated that claim and found it to be false - would the 3 reporters have been charged with perjury? given that there is no underlying crime here, those lies would have been just as pertinent as Libby's. Fitzgerald didn't investigate that, and would never have charged them.