That clause is place because it is a felony in the US to export some classes of software to some Countries. Any contract that requires a signatory to do something illegal is null and void.
So a contract signed by someone to break a law (eg commit a murder!) is not legally binding. Similarly a contract (the GPL) which requires you to break the Law, (ie export some software to Iran) is not legally binding.
I'm not quite sure of your complaint about people stealing GPL'd work in Iran though. If the concept of copyright is not even recognized by Law what hope do you have of preventing people from using your work without attribution or compensation?