There are places where firms are compelled by regulations to hire people who are currently US citizens, sure. That's not really employer-discretionary (and I really should have said ethnicity or national origin, and not nationality, anyway.)
But even then, it is not, as suggested upthread, more OK for Iranians to hire Iranians than for Americans to hire Americans, whether talking about nationality, national origin, or ethnicity. (Fir nationality, there are narrow cases where the latter is mandatory and the former is prohibited no matter who is hiring, sure, but that's the opposite of what was suggested.)
