Why wouldn't the corporations have a copyright interest in the book their editor helped shape? If book editors can't have a copyright interest then neither can re-mixers.
I'd think that the issue would boil down to the nature and extent of the editing. Typos and grammar? Not so much. But re-ordering and restructuring multiple narratives into a coherent whole? I'd have to say, yeah, that is deserving of a copyright interest.
While I am offended by the Anne Frank Foundation's last minute copyright maximalist control and money grab (they really should have planned in advance for the known expiration date of their cash cow), there is merit to the idea that structural editing should confer a copyright interest in the altered version, but not the original material. It is certainly not a "bizarre" claim, but rather one that BB has likely made repeatedly with respect to re-mix culture. Life of the author plus 70, though, remains complete BS.
(OT: way too easy to accidentally delete a post. No confirmation button when I accidentally clicked on the trash can instead of the pencil. No undo/undelete function I could find.)