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In the US there is no way to release it works into the public domain (unless you work for the US Government). If you wanted to be free-er than "all rights reserved" you must give it a license that says that. Without a explicit license all new works are automatically assumed copyrighted by their owner.

http://en.wikipedia.org/wiki/Public_domain_in_the_United_Sta...

In short it's not always clear that you can voluntarily abandon copyright in every countries that follow the Berne convention. It's safer to grant a liberal license.

The only problem with that is that in most jurisdictions the only way to release something into the public domain is to have died seventy years ago.