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

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.