The plan is to auction them as seized property.
That doesn't sound very nice.
What if the person that buys them get's them at a favorable price and decides to dump all of them at once when the price is good enough for them?
Price crash once again.....

I think they should just destroy them

Much better for mankind!
That's just what is done in the War on Drugs. Blame Reagan-era laws for that.
Also blame DPR/Ross Ulbhricht for being dumb enough to leave a hot wallet with $80 million of his personal stash of bitcoins on the same laptop that he used to administer Silk Road, only an expensive password attack away from seizure. Otherwise the feds would have had only $20 million.