Karl Rove could feed them false docs all day long. It remains the responsibility of the network to check it out before they put it on TV.
They did check them out. They had sufficient information to debunk them, but ran the story anyhow thinking "nobody will question us. We are 60 Minutes."
They are now merely using the CBS Legal Department's formal response (preponderance of evidence is a very precise legal term in libel law) so that they don't wind up in a lawsuit.