Some countries don't recognize the concept of Public Domain works. In the US, many government works are Public Domain as a matter of law. This creates complications internationally in those countries that don't recognize the legitimacy of Public Domain as a legal concept. Nonetheless, the US still wants to make it available internationally.

To satisfy these conflicting requirements, the US government places it in the Public Domain in the US to satisfy US law. Additionally, they make it available internationally under a license that approximates the intent of Public Domain while still being recognized as a legally valid thing.