It's not immediately clear to me that this is illegal. If anything, the Feds didn't "seize" the Bitcoin, they forcibly acquired a license to the wallet's private key. If they didn't understand that the license was non-exclusive, well, that's not the fault of the other licensees.
Gary is accused of money laundering and other crimes, because of the money laundering and other crimes. Money laundering is a tacked on charge that requires discovering an illicit origin of the funds. Then subsequent actions, such as the laundering through additional mixers, depositing in BlockFi for collateral with clean funds loaned against the dirty ones, and using those clean funds, can all be criminalized.