It shouldn't come as a surprise that Iranians manage to still get Apple products despite the ban. What's slightly more shocking is the number of stalls that openly hawk the merchandise. For something referred to as "the black market," one would think that they would be a bit less brazen.
The sanctions stem back to attempts to halt any progress in Iran's nuclear program -- which they insist is completely peaceful and nations clearly never have ulterior motives. Unless given special authorization, U.S. companies are unable to sell goods or services to Iran. The sheer amount of Apple gear on display in Tehran indicates just how effective said sanctions are.
There's even a company called Apple Iran, with a strikingly similar website save for the Persian language, that U.S. Apple has been attempting to shut down. Apple Iran and other firms still manage to have the company's newest models shortly after they're released even with all the focus on them from governments and private corporations.