Indeed it is not valid in most places, as things become public ___domain when copyright expires but you cannot give up your copyright. Carrying round eg a BSD license declaration is more useful for this reason.
I'm curious if there is anyone in a place that doesn't "recognize" an author's right to give their work to the public that's actually bought a license?
But I also think there were issues with Public Domain isn't exactly a licenses and acceptable in every country.