Many Iranian-Americans still seem confused by a rumor that the US sanctions law makes it illegal to buy Iran Air tickets. It is NOT illegal to buy Iran Air tickets. When asked about the matter, I refer friends to the NIAC clarification dated July 1 2011 which says, “Iranian Americans may continue to travel using Iran Air, despite the recent imposition of sanctions on the company, according to the U.S. Treasury’s sanctions enforcement agency and the U.S. State Department.”
The confusion may have arisen due to the fact the the Iran Air sanctions do prohibit other types of transactions with Iran Air, such as transactions related to the provision of insurance, aircraft servicing, or the export of goods to Iran Air. Full text of NIAC clarification, here.
I do recommend checking the NIAC site (and the date of the clarification) every time you plan to buy a ticket.