Photograph taken by an employee of the US Federal Government and copyright law.

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This question needs to be parsed into two parts, first ownership of copyright, and second, law of privacy. First, you are correct that the U.S. Government cannot own a copyright. Therefore U.S. Government cannot create a copyright by fixing the subject matter in a tangible medium. Thus, if an employee of the U.S. Government takes picture while acting in his or her capacity as an employee of the U.S. Government, said picture has no copyright protection. Also remember that the copyright is created by the author (photographer) once the subject matter is fixed in a tangible medium. The subject of the photograph would not obtain a copyright be being the subject of the photo. That subjects rights are governed by the law of privacy.

The rights of the individuals in the said picture could turn on the law of privacy. Private individuals have the right to control the use of their image. There are, however, exceptions to this rule. For example the law does not apply if the picture is a matter of public interest, such as car accident, or fire, or war footage, typical images used in news reporting. Also public figures, such as politicians, actors, and sports figures rights are limited. By thrusting themselves into the public sphere, their images are a matter of public interest and therefore they have less control than a private individual, but it is not zero control either.

The examples provided in your question, pictures of war prisoners in compromising positions, certainly could be considered a matter of public interest. As such, they would not be protected by the law of privacy. Also the law of copyright is a law designed to provide remedy for infringement, and not just control. U.S. Government does have a right to control its property, even if it is not protected by copyright.

I hope that this is helpful.