I worked in TV for ten years. I'll take a shot at this...
When this clown leaves ABC, his personal journals, if he was smart enough to keep one, are worth millions. Also, they don't want Fox to be able to promote their latest hire as 'the brains behind Nightline" (as unlikely as that might seem, it could happen).
Another consideration is that this guy could scuttle Nightline on his departure by poisoning the 'guest pool', either by taking his Rolodex to a competitor, or, equally likely, telling everyone he's ever dealt with to ignore Nightline and go to [insert media here].
Use your imagination along those lines and you'll get the jist of why ABC is playing nice...
Sounds like a fine line to me...