Actually, a lot of criminals also uses crypto for their payment especially in the black market. Why? Because it is hard for the authority to track all the transactions, and if they do, how will they locate the criminals? having fiat money is more risky because the authorities have a money that they call "Marked money" where if they saw it, it means that the person who is using it is a criminal or maybe they used the mark money for the payment. But before that happens, usually the criminals will get caught.
In my opinion, it depends on what kind of transaction they were doing. If it is in an online black market, they will use crypto, but if it is a transaction between two party, they will use fiat money.