In the case of the USA, I don't really understand it.
It’s the same as Germany. They have rules and processes setup to deal with confiscated assets. They can’t just decide to keep something if it may go up in value. I imagine it would easier to auction the BTC no matter what and try to pass a bill to begin buying BTC then it would be to just move the confiscated BTC into a reserve fund. Especially when you consider that a lot of that BTC was stolen from actual victims who probably want to fight to get it back (see: Razzlekhan).
Ok didn't know that. And after all we wouldn't know if they have accumulating BTC for a long time. If the US government knows that countries like Russia and Iran and North Korea get into BTC big time, it is hard to imagine that they would not acquire a decent amount of coins over time for themselves without letting the public know. Would they have to let the public know in theory?
Of course if the victims are known or could be identified at some point in the future, then you are perfectly right that they can't just keep those coins for themselves. But my guess would be that that a good amount of those confiscated BTC come from drug dealers. What would the correct legal way then be when someone sold drugs for $10,000 in BTC, gets caught, and in the meantime those BTC increase in total to $100,000 until the case has been settled? Put aside the fact that someone might have to go to jail, but would the drug seller have the right to at least get back some of the value (after paying a penalty)?