Here's the CBS statement on Rathergate.
Now, why is CBS protecting the source? My guess: Because whoever it is, the source is important to CBS. If it were simply Mr. Burkett or some other small fry, then CBS would burn him or her mercilessly. If, however, the source is one CBS will need in the future, CBS can't burn it.
Hmmm. What kinds of sources does CBS need to avoid harming? The Democratic National Committee and the Kerry campaign come to mind.