The images are almost certainly in the public domain under current U.S. case law. Releasing them under CC0 is a way of saying, “just in case they aren’t in the

The images are almost certainly in the public domain under current U.S. case law. Releasing them under CC0 is a way of saying, “just in case they aren’t in the public domain, now they effectively are.” I don’t know what you mean by unenforceable—what is there in CC0 to enforce?