The core thing about licenses, in general, is that they only grant new usage. If you can already use the code because it's public domain, they don't further restrict it. The license, in that case, is irrelevant.

Remember that licenses are powered by copyright - granting a license to non-copyrighted code doesn't do anything, because there's no enforcement mechanism.

This is also why copyright reform for software engineering is so important, because code entering the public domain cuts the gordian knot of licensing issues.