[Doctor_Fegg]

Most open licences work by manipulating copyright. If you entirely relinquish the rights (a la CC0 or other "public domain" declaration), you can't enforce attribution or share-alike, which OSM wants to do.

[sigg3]

It doesn't manipulate copyright, it presupposes it.

The 'left' part of copyleft is about distribution or dissemination, and other rights bestowed. It doesn't challenge copyright at all.

You cannot grant rights for works without ownership, so PD has left the chat, so to speak.