It's also fun that, because this is from the US, they can't just use CC0, but instead need to clarify that this must be public domain, separately from the worldwide CC0.
Less snarkily, I do wonder about the discrepancy there.
Creative Commons is a worldwide organization, not a jurisdiction-specfic organization. Creative Commons does not have the authority to harmonize laws worldwide.
PS: AFAIK, however, Authorship rights are different from Copyright, and cannot be given/passed as Copyrights, at least in US.
Also it's important to remember these works are not in the public domain because someone declared them to be, they are simply because they are works carried out by the US government. Similar to how copyright is automatic, it's not applied only when you put the copyright symbol, that's just informational.