> You can't just bypass the whole "make a rule" and "show that $X broke the rule" thing and go straight to "punish X".
Yes, as long as the punishment isn't essentially criminal in nature, you can, and it's long been a regularly used tool of foreign policy, under the International Emergency Economic Powers Act. When sanctions are declared against foreign companies or individuals or trade with them otherwise restricted, it's based on declared threats to the country, not general rules that are announced in advance and then the specific individuals involved found guilty in some judicial process of breaking after the rule was declared. It's all by executive finding.
That this particular banning may be legal (and I think that's arguable) doesn't make it good policy (which I strongly believe it is not), because the reasoning behind nulla poena sine lega applies here too. The whole reason we have rules and not a chaos of capriciousness is to make it possible to plan, predict, and invest. A state that arbitrarily punishes international parties for unclear reasons will decrease confidence in all foreign interactions.
Yes, as long as the punishment isn't essentially criminal in nature, you can, and it's long been a regularly used tool of foreign policy, under the International Emergency Economic Powers Act. When sanctions are declared against foreign companies or individuals or trade with them otherwise restricted, it's based on declared threats to the country, not general rules that are announced in advance and then the specific individuals involved found guilty in some judicial process of breaking after the rule was declared. It's all by executive finding.