IANAL but a proud American haha, and we're very specific on not allowing ex post facto laws per our constitution. It would be a huge avenue for abuse of power by the government against the people.

The caveat I added to my initial comment that you also mentioned was that they could try to find a relevant existing law and retrofit it, e.g. general securities laws, and say that this is a securities market and so this has always been illegal — but it's very unlikely and I doubt it would work. Far, far more likely that we pass explicit laws about this.