Because of long standing history of coups, espionage, nuclear sanctions and everything else in between that is outside of the average American citizen unconnected to the historical actions of their government, all US passport holders are only allowed into Iran if part of a tour group (or with a guide). It apparently still stands that Americans must also be escorted from the airport to the hotel, and cannot make their own arrangements (it all has to be pre-organised – a simple addition via a tour company).
