Tim Starling wrote:
"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."
For those who may be unconvinced: the ADA Reference Manual shows a
Copyright (c) Department of Defense.
Reason: the manual was written by a contractor, which assigned copyright
to DoD.
(Note, however, that the US seems to let people use its copyrighted
works royalty-free, effectively under a "free license".)