I think ABC has decided to try and keep a little credibility for itself and for the MSM and not go down with the CBS ship.
Plus, they may have realized it's over for Kerry and have decided to bail.
I think ABC is enjoying this 'death by a thousand cuts' to Dan Rather and the CBS News team.
ABC and the other MSM smell blood in the water and are going after CBS...they are fighting over the same shrinking pool of viewers/readers. This has little to do with credibility (they have none) and more to do with plunging the dagger deeper to a competitor.
GO, ABC, GO!