The National Air & Space Museum paid for the model right? They should to do as they wish. At least they are displaying it instead of storing it in box. What gives the government the right to take the model away from them?
Umm, it is the National
Air and Space Museum. It, and the rest of the Smithsonian, is a body of the federal government. The government wouldn't be taking the model away from anyone; it's already theirs. Or rather, as plynch
said above, it belongs to the American people, and the government, through the Smithsonian, takes care of it on our behalf.
And no, they didn't pay for the miniature. It was donated to them by Paramount in 1974.