I heard or read somewhere that it would be an infringement on 1st amendment to ban and in my opinion even regulate cryptocurrency as it's just a sequence of numbers and letters. You can't ban code. Where does it end? I feel like if this became a thing and the govt took steps to ban crypto the ACLU and all kinds of civil rights groups should step up and defend crypto users.
In some countries, any currency other than the national one is prohibited, because of this, not everywhere can regulate. There is another option is monopolize bitcoin, but this is impossible, so it does not work out that way.