The point is that CBS doesn't care if they were duped. They got the story out that they wanted to get out, and as long as CBS itself didn't forge anything, CBS knows that they haven't made themself liable, so they don't care.
They are perfectly happy to do bad reporting that relies on someone else's lies and deceptions, because it gets out the message that they want out there, and they are not legally culpable.
We don't know that they didn't......
It doesn't matter about "liability"--if this is indeed a forgery, it will be an incredible embarrassment for Rather.
Here's hoping.
I would submit that there is a due diligence standard that they need to have met, and IMHO they didn't (even I would have know to question the modern appearance of 32 year-old memos).
If I'm right in a legal sense, they were therefore negligent.
Whether the negligence is actionable is another matter, but if I were CBS I would at least be working up a bit of a sweat right now.