[yellowapple]

Public domain code ain't GPL-compatible though unless it's explicitly released under a "license" that functions as a public domain grant (e.g. CC0, 0-clause BSD, WTFPL, etc., though WTFPL has some legal wording issues IIRC), the reason for this being that not all jurisdictions recognize the concept of "public domain" and thus require explicit license terms.

[wolfgang42]

Wouldn't the PD "license" in this case be US law [1], which specifically prohibits copyright protection for works produced by the government?

[1] 17 USC 105: https://www.law.cornell.edu/uscode/text/17/105

[janc_]

I don't know any jurisdictions that don't recognize the concept of “public domain”, but there are some that define “public domain” differently than it is defined in the US.

In the US “public domain” means “without copyright”, while in some other countries it means “without authors rights” (authors rights are a superset of copyright). The problem here is that some authors rights (both in the US & in those countries) are “inalienable”, meaning they can't be given or taken away (although there might be exceptions listed in the law).