I would think that anyone who knows a little about how the U. S. Government works would have guessed quite accurately when and how the auction would take place. They must dispose of any property that they seize according to the law. They had neither the choice nor the motivation to keep the coins, trade with them, or sell them in a private deal.
Couldn't they just "destroy" them, as they do for illegal stuff like drugs ? (send them to a random address and not store the private key)
The fact that they sell them instead of destroying them has a strong meaning.