Challenging Smithsonian's Copyright Control

The Associated Press reports today that a nonprofit group is challenging the Smithsonian Institution's policy of claiming the right to prevent people from reproducing images under its control. The group is posting thousands of the images on public photo-sharing sites. I was struck by this part of the AP story:

Images made by federal government employees are exempt from copyright law and are considered to be in the public domain, said Robert Brauneis, a George Washington University intellectual property law professor. Most Smithsonian employees are considered federal workers because their salaries come from taxpayer funds.

So I ask again: How can the National Park Service claim it has exclusive rights to an image of Lewis and Clark that it created that was later used by a book publisher?