"We the people" source released

"Copyright protection under this title is not available for any work of the United States Government, but the United States Government is not precluded from receiving and holding copyrights transferred to it by assignment, bequest, or otherwise. - 17 USC S105

Further, something doesn't need to be copyrightable under domestic law to be considered copyrightable under Berne. The US government claims the right to assert copyright over its works outside the US.