Sources don't have to sign any type of non-disclosure and aren't prohibited from speaking. Basically they're being interviewed for their knowledge on something, contacts they had while overseas, conversations, etc.
They're not participating in operations, aren't employees, just people who have information. They're not under any obligation to not speak about their info or experiences.
Where Wilson hangs himself, though, is the Niger forgeries. How did he find out about them? Did he have security clearances to receive that information?
He now admits he didn't see them prior to his trip and didn't even mention them during his interview with the reports officer. But even if the agency knew about the forgeries before having them in hand, and that prompted the request from the VP's office (which I doubt), that's not information he would have been given before his trip.
Nobody is asking Wilson how he knew about the forgeries, whenever it was that he actually found out. I think people assume that he would have been given that info as part of his "mission", but you don't give sources classified info. You just interview them for what THEY know.
Cheney's office denies ever giving him this mission. This was purely a CIA op. That's what got Scooter going. WHY WOULD JOEY CLAIM CHENEY SENT HIM WHEN HE KNEW IT WAS NOT TRUE?