All currency are used in Illegal activities whether it is bitcoin or USD or any other form of money because it is used to buy things so that it used for illegal activities such as gambling, buying drugs and other. As long as it is form of money it can be used into Illegal.
I agree, and not only USD and Bitcoins, all the currencies of this World which are in the form of black money specially is being used for illegal activities and corruption is as well increasing.