While a work may be in the public domain in the U.S., it may be under copyright elsewhere. So, e.g., while works by the U.S. government may be public domain in the U.S., they may remain under copyright in other countries.
Damn. Did some more research, and you appear to be correct with respect to the most recent interpretations of the law. :-P The current interpretation of 17 USC Sect. 105 is that such works are copyright-controlled in countries which have copyright control over the works of their own governments.
However, the U.S. government may license their works and thus give permissions with respect to these foreign copyrights.
Traditionally, the US Government has not abused this foreign copyright opportunity, and has treated its works as public domain worldwide. Accordingly, it hasn't issued explicit licenses. See for example from the USGS website, on http://sfbay.wr.usgs.gov/access/copyright.html: "USGS-authored or produced data and information are in the public domain."
The current interpretation of 17 USC appears, frankly, to be another example of encroaching copyright where it doesn't belong. :-(