Not sure about US law (maybe D&T can comment?), but German law is crystal clear on that. If someone accepts money in good faith (as part of a legal transaction) then he owns that money. It is not possible to legally acquire stolen goods, but money is an exception (§935 BGB).
That is my understanding. Good faith is the important part. Obviously it doesn't protect someone who knows (or should have a reasonable suspicion) that the money is the proceeds of a crime. Doing otherwise would undermine the confidence in the currency, much like the OP proposal would fatally undermine the confidence in bitcoin. If a merchant, user, or exchange can accept Bitcoins in good faith and then have those coins rendered worthless through no fault of their own .... why would they want to accept a bitcoin? Even if they are interested in crypto-currency they would just support a coin without that risk.