If I had to guess, my assumption is that they "verified" the document as authentic, and then prefaced it with some (lightly) worded disclaimer, then run with it. (Still has legal denyability). This way, they can act surprised when the authenticated document turns up to be false.
"Honest, we didn't know that this document was fraudulent."
I bet the lawyers had their teeth in it before anything went on air, and said that any trouble that brews they would handle it.
Not so much fraudulent but misrepresented.........no excuse can protect CBS.........