I think you have missed out a little detail here. All works that you create are always copyright. You can disclaim copyright and make them public domain in which case anyone can do whatever they like with it. Anything in between is still copyrighted but licenced for certain usage. If you licence something for creative commons (CC) then you still are the copyright holder and can licence that work in another format too which may be less or more restrictive than the CC. Then a user can choose which licence he wants to abide by out of the options that you have given. What a CC or GPL licence says is that although the work is copyright you expressly allow certain use with certain strings attached depending on the wording.
So why no go public domain ? Well once you disclaim copyright and anyone can do anything then it is often the case that the image is put on web sites which say that the web site is copyright. Ok thats not an issue but then time goes by and eventually when the only copy you can get is from copyright sources then there have been cases where its hard to prove that the originals copyright has indeed been disclaimed and then basically the artwork becomes hijacked.
By using a licence the user of the work on a website should honor the licence terms and put up the CC banner or whatever otherwise they risk being whacked for non compliance.
The theory is that the web site owner should be liable to prove copyright for all the stuff on it but in practise if the web site is a big corp (like Disney etc al) then it basically means that you have prove the public domain-ness of the image before using it. Even with very very obvious cases of copyright expiry the situation has become so nonsensical that the public domain stuff leaves users of it exposed to infringement cases.