This gives the US government a lot of power over bitcoin... That both gives the government an incentive to let bitcoin thrive (to protect it's holdings), but also an opportunity to substantially manipulate bitcoins market price, possibly down to nearly zero. Or the keys to these bitcoin will sit idly in a filing cabinet forever...
Despite loads of blockchain haters on HN, the government recognizes that bitcoin has value and has formal processes to seize, custody, and auction off the assets to add money to the federal treasury.
All this means is the government is aware that bitcoins can be traded for real dollars. Its job when seizing assets is to simply perform that transaction.