Why Can't We Use Military History Photos in Book?

Dear Rich: We publish ebooks for the Amazon Kindle and other ebook devices and frequently use U.S. government material for our works. We are very clear on what to use (e.g. no contractors work) and where to distribute it. But one thing appears very strange to me. How can the U.S. government say that the information on a certain .gov or .mil website is public but at the same time restrict commercial usage? To give you an example kindly check this site that offers military and historical texts worked out by U.S. government officials and not registered at the Copyright Office (we checked that already). From my understanding - and that's true for Germany - if a "thing" is public domain you are free to do with it whatever you want, give it away of sell it, make derivatives etc. etc. Can you shed some light on this? According to the CMH site:

"Unless otherwise noted, information presented on CMH Online is considered public information and may be distributed or copied for non-commerical purposes. Use of appropriate byline/photo/image credits is requested. If copyrighted or permission restricted materials are posted on CMH Online, the appropriate credit is given. Visitors wishing to repost or use such materials for their own projects should make separate arrangements for permission with the owner."

In other words, the site claims to sift out those works that are under copyright by labeling them with a credit. Everything unlabeled presumably is in the public domain. Assuming you can trust the site's filtering of material, then, as the Supreme Court has stated, you can do anything you want with those materials, with or without attribution to the author.
By the way, although works prepared by federal government employees are in the public domain, you may be surprised to learn that the U.S. government -- though it rarely exercises the right -- is legally entitled to claim copyright outside the U.S. (see page 59 of link).
P.S. The licensing dilemma. Finally -- and this doesn't seem to be an issue at the CHM site --  we're always concerned about the trend to license public domain material. For example, if you check out the terms for this Department of Defense site, you'll see that some restrictions have been placed on use of U.S. government imagery. That claim is made on the basis of a license --  that is, you agree that as a condition of using the website, you will abide by the rules regarding photo use. Generally such licenses are only effective if the user must click to accept or demonstrate some action taken to indicate assent.
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