I read this theory on another thread....McCain probably did tell the Bush campaign. Rove decided it would end up being a "he said, she said" kind of thing. MSM would, of course, back kerry and that campaign would, of course, deny, deny, deny. Might have cost Bush valuable votes.
Kerry wanted to identify himself more closely with McCain to appeal to certain independents and supposedly republicans. Therefore, contacts with McCain would help Kerry. In the final days of the campaign, Kerry was constantly saying he would reach out to Bob Dole and John McCain, as if invoking their names meant he had their endorsement.