Why doesn't the U.S. Federal Government auction their retired but memorable aircraft?
There are a few reasons for this. 

First off they may not want the public, or more importantly potentially enemy nations obtaining a large quantity (or any quantity for that matter) of military planes. Even old war birds can still do a lot of damage if equipped by a person with intent on doing damage.  

Secondly, the boneyard you have pictured serves a few purposes. For one it houses aircraft that have reached their life time limit. To keep an airplane flying it takes not only money but a supply of parts (or the resources to make your own under certain FAA rules). Out of service planes may be tough if not impossible to keep flying for no other reason than you cant get airworthy parts for them. 

Thirdly, some of the planes in these bone yards are scrapped (or kept there) for reserve parts for the planes still in service. Again airworthy parts are not as simple to get as going to your local car parts store. There are many more regulations around them. In this case you can pull parts from an airworthy plane (if you know the cycles its been through) and use them to fix another plane.

Lastly, and this is mainly speculation, maybe the government is hidings its back up planes in plane sight...

On the note about buying old war birds, rules have changed over time and its not as easy to get ahold of government property as in days past. Some military equipment that sees its way to the public hands actually left the US as a sale to another country then that country sold it into public trade as well. For what its worth Trade-a-Plane has a whole section just for war birds.