The Whitehouse has been posting the official photographer’s photos on flickr at http://www.flickr.com/photos/whitehouse .  They each have the following written in the description:

This official White House photograph is being made available 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 or used in materials, advertisements, products, or promotions that in any way suggest approval or endorsement of the President, the First Family, or the White House.

I know there may be some exceptions based on the Copyright page the photos link to, but I thought everything (not falling under an exception) published by the US Government was released into the public domain.  I first looked on Wikipedia to get background info.  I started with the Public Domain page.  Then I moved on to the Works of the US Government Page.  So far, my suspicions seemed to be correct.  This Cornell page backed the information up.

So if works created by the US government are in the public domain (in the US), then there are no copyright protections.  And if there are no copyright protections, no one owns the rights to the work.  Therefore, how can it be that the photos may not be manipulated in any way?  Any work that’s in the public domain is free to remixed in any way possible.  In fact, it can be combined with anything for any reason, including advertisement.  So is this just a matter of the government trying to exert control based on the assumption that most people will be too lazy to look up that government works are in the public domain?  Or am I missing something here?