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.
Depends in the laws in question and whether it really - needed - to do something around said laws..
As I remember, in Germany, for example, there are laws that directly state that what was seized should be converted to fiat eventually, and it was done with BTCs their governement seized, even though it would be better for it to be otherwise in the long run.