From Marginal Revolution:
By applying a dual citizenship provision, in effect we are making Iranian law American law. It is Iran who determines who is banned, not Trump. You even could imagine a foreign government using this to punish or blackmail people who have scant current connection to their nation. What should I do if Yemen offers me honorary national citizenship, in return for the service of promoting their cuisine and restaurants in the fine state of Virginia? Can I turn it down? Prove I don’t really hold it? What exactly is to count as such proof?This is a rather good point about the complexities of immigration law. Dual citizenship is always going to be a complex things. But it is a fair point that this puts control over border crossing with governments that are not always close friends and allies of the US government.
Now one presumes that this sort of "targeting by citizenship" could be fixed in an actual court of law. But it does speak to why complex regulations can make sense -- to minimize gaming and to provide clarity for complex cases.