Why does the White House claim to impose terms of use on official photography?

The White House has a Flickr photostream for disseminating official photography, including the excellent work of Pete Souza.

I’m confused, however, by the following statement that appears below the photographs:

This official White House photograph is being made available only for publication by news organizations and/or for personal use printing by the subject(s) of the photograph. The photograph may not be manipulated in any way and may not be used in commercial or political materials, advertisements, emails, products, promotions that in any way suggests approval or endorsement of the President, the First Family, or the White House.

The stipulation that I boldfaced appears to be incompatible with the fact that U.S. Government works are generally not protected by copyright. Indeed, the Flickr page indicates that the license is United States Government Work; that official government web page specifically states that “Anyone may, without restriction under U.S. copyright laws… create derivative works”. (The USA.gov page is consistent with the Flickr page in prohibiting the use of works to suggest endorsement.)

The claimed prohibition against manipulation is of immediate relevance, since an Orthodox Jewish newspaper published a version of the Situation Room photo with Hillary Clinton removed.