I can see Space X's point here: federal law restricts them from releasing certain information and technology to certain countries. If you hire an employee from that country and grant them access to those items, it's tantamount to releasing that information to the restricted foreign country.

If the answer is that us labor law overrules the export control law, then so be it, but I don't see this as a case of illegal discrimination so much as misunderstanding how multiple laws interact with each other.