"The code represented here is the sole property in origination of acts commissioned by the United States government and thereby not owned or copyrighted by anyone else at the time of original posting unless documented within the code listed."
Now the difference between this type of code with your license verses windows or mac or anything else is that they have already asserted copyright over the works in question. All we have at this government site is works claimed not to be copyright-able or public domain in which you couldn't attach a license to legally without otherwise expressing ownership in some way. Declaring it's origination as a public domain stops anyone from copyrighting or trying to and claiming to be the originator of the code (to sneak in a copyright in a foreign land that by treaty could be asserted). Or in other words, it is completely different to claim I own something you have already copyrighted than it is to assert a copyright on something without a copyright yet. So with international copyright rules (even US rules), an implied copyright is granted at origination of the work and this would specify the origination preempting others from trying to do so.