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.
Another way of saying this: Creative Commons, based in California, USA, did not publish a license that can be used by one of the largest domestic authors of software.
Less snarkily, I do wonder about the discrepancy there.
Category Error.
Public Domain is not a license. It is a state of being.
Creative Commons is a worldwide organization, not a jurisdiction-specfic organization. Creative Commons does not have the authority to harmonize laws worldwide.
In other words, think of Copyright. A Copyright holder can apply any license they like, and change the licenses for new versions whenever they like. Public Domain is explicit forfeiting the Copyright, which means authors cannot enforce any license (and anyone can just take their work and declare it it's theirs, apply licenses etc).
PS: AFAIK, however, Authorship rights are different from Copyright, and cannot be given/passed as Copyrights, at least in US.
It's not that they can't, it's just pointless to offer a licence for something that's in the public domain.
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.
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.