the laundering wouldnt be all that hard to take those buttcoins on in exchange for clean ones.
Because all the exchanges they'd use are regulated in some form and at least adhere to anti-moneylaundering-directives and boycotts, or they can't operate in the US and/or EU.
Sure, they could move to p2p exchanges or exchanges in unregulated jurisdictions, but that requires quite some effort and thought. Which, by definition, would exclude them from being "idiots".
edit: I suppose it would be hard still. You can get anonymity inside wallets with Tornado Cash but then to turn that into actual money it will be very dependent on the country