Licensing, and particularly copyleft is based on copyright - you cannot offer a license, if you don't have a copyright on the thing. You can put it in the public domain, but that is very different.
I understand that. It was just curious to me why, if one holds the position that information ought to be as open as possible, the author still chooses to copyright their won writing. It seems to me, that the ideal is putting it in the public domain, i.e. no copyright. But maybe I'm missing something.