Anything seized becomes the property of the government and they have to decide what they want to do with it. If they have a place in their constitution that specifies what seized assets will be used for then they should follow their rule of law. If you're looking from the moral angle then you can argue that the US government did not directly buy the seized cryptocurrency so they shouldn't keep it in their reserve. But others can raise a counter argument that immediately the coins were seized they automatically becomes a US property and they can choose to keep it. Anyway we don't know if the government has a timeframe to offload the seized coins in the near future, we'll wait and see.
This is wrong though, if a coin is seized then it means the person that received the coin the first place did it illegally but I want believe that the coin has an origin, why not send back the coin to the original owner. I can't imagine someone coin was hack and lose everything they have fault for only for another country to hold into it. The only coins that government is supposed to keep is only when someone use it for terrorism or use to trade illegal goods.
I remember how silk road Bitcoin was sieze years back, he even went for prison for it because it was reported that the Bitcoin in his possession were gotten under the influence of dark web and any activities on dark web is illegal, he was later pardon but all the Bitcoin was taking by the government which was understandable, he got back though because people still donate some Bitcoin for him after his release, that's one power message of a decentralized community.
Since it was been seized that means those coins are came from illegal activities.
Also US didn't seize those coins immediately since they undergo with lots of process. Much better if they seize those coins since somehow its helpful for them, also that's their law and part of consequences to those which did criminal activities in their jurisdiction since this serves as a warning that never do any financial crimes and terrorism in US.