...to allow people to withdraw their coins?
Once the government identified Ross Ulbricht and had control of the Silk Road servers, why didn't they keep the site open so that people could withdraw their bitcoins? They could have blocked the ability to trade (so as to not facilitate any drug deals) and instead only allow people to log onto their accounts at the site and withdraw their funds to their own Bitcoin addresses.
How would they 'let' them do that? Surely they'd have to make an announcement and obviously that's not really possible without giving the game away. Besides, they likely saw all the money there as confiscatable seeing as it was being used for illegal activities so wasn't in their interest do do what you propose.