For large, conservative investors, this is the first chance to buy bitcoins directly from the U.S. Government. There is an implicit guarantee that the coins to be sold are legal and without any sort of taint, at least in the eyes of the Feds. Also, to many such investors, the U.S. Gvt is the most trusted trading partner possible. They are the most reliable organization to trust wiring millions of dollars to.
The auction is a giant stamp of approval, if not for bitcoin as a whole, than at least for the particular set of coins at auction.
There are obviously reasons why the coins may sell at a discount as well, and I tend to think that is more likely. My guess is there is about a 20% chance the coins will sell over spot. After all, the supply of government-approved bitcoins is very very tiny.
It is a tricky one to value the coins could well sell at a premium relative to the Bitcoin price at announcement
In other words if the market drops 10 to 15% on this news the premium would be if the Bitcoins were bought at the market price before this release
That or they offer market rates, as for your point on the taint it's a good point that in the opinion of the government these coins sold through legal auction are seized property and now they are being resold legally so there is no issue about them being illegal.
So these coins could easily get a premium if a company is interested in them.