You can be blamed for the source your Bitcoins came from. The thing is, you've sent your BTC to a third party's wallet. It was basically not yours anymore, they had the right to seize the funds.
I'm not a lawyer, but my common sense doesn't completely agree with these statements.
If you compare this situation with a fiat analog, it would be more or less an innocent person that received money in an honest way, but said money seemed to be stolen a couple months prior. The innocent person brought this money to the bank, the bank accepted the fiat money, but afterwards changed their database so the funds were credited to law enforcement instead of the honest person.
I do not agree that the money was no longer property of the honest person when he gave it to the bank... He was no longer IN CONTROL of his money, but it was still his money.
Neither do I think it to be fair if the FBI would seize the money... Sure, they can ask the bank to FREEZE the account during investigation, but as soon as they found out the person received the money in an honest way, they should unfreeze his account asap.
This whole thing reeks....
Either the op is trolling, he received the money in an unethical way, he"s getting scammed, or law enforcement is acting in an unethical way ( I've heard similar horror stories about American law enforcement tough...)