There's no such thing, no law that allows one to place their work in the public domain! Yes, you can attach a copyright license that allows one to use your work for any purpose, but you still retain the copyright.
'Examples for works actively dedicated into public domain by their authors are reference implementations of cryptographic algorithms,[6][7][8] NIH's ImageJ,[9] and the CIA's The World Factbook.[10] The term is not normally applied to situations where the creator of a work retains residual rights, in which case use of the work is referred to as "under license" or "with permission".'
The prohibition on copyright protection for United States Government works is not intended to have any effect on protection of these works abroad. Works of the governments of most other countries are copyrighted. There are no valid policy reasons for denying such protection to United States Government works in foreign countries, or for precluding the Government from making licenses for the use of its works abroad.