Park Service Lays Down Logo Law

Ben Long of Kalispell, Mont., wrote a book several years ago called Backtracking: By Foot, Canoe and Subaru Along the Lewis and Clark Trail. But now, his publisher, Sasquatch Books of Seattle, is being forced to recall and destroy paperback copies of the book that have been sent to bookstores. Why? Because the publisher included a silhouetted image of Lewis and Clark on the cover that the National Park Service developed. The Park Service, the Associated Press reports, demanded that Sasquatch Books "cease all use of this official artwork."

Can someone with a legal background explain this to me? This logo was created with taxpayer dollars. The Park Service is part of the federal government, which, after all, is the representative of the people. So how can the government own this and other images and logos independently of its citizens? I can see why the Park Service and other agencies don't like people profiting from their work, but I don't understand how and why they can stop it.