They can't "sanction all forks". Sanctions are applied against people and organizations, not technologies. Criminal money laundering is still a crime. It doesn't matter if you use TC or not.
The "if the U.S. government publishes smart contracts, I win" --Vitalik Buterin or whoever snark may not be as far fetched once tooling is bettee understood.
Humorously enough, ATF already has more than enough experience at that of enforcement (surveiling new designs and classifying new works as NFA/non-NFA) for it to not be converged upon as a potential model for regulation of such logical constructs.
Throw im some aggravating statutory enhancements for any crimes to which cryptocurrency mixing is found to be a component, and you've got a recipe for at least ensuring normal folk in U.S. jurisdictions tend to stay away from it.
...Good God. I hate this. Please don't make me right on half of these guesses.