"Since one of the United States' illegal actions disclosed by you is spying on the Iranian citizens, we invite you to visit Iran and elaborate on the US administration's measures in this regard in detail," the letter said. NGOs in Iran are, typically, closely affiliated with the government. U.S. cyber-espionage against Iran has frequently targeted the nuclear program there, including with a program code-named "Olympic Games" that sabotaged uranium-enrichment facilities.
Snowden has previously leaked information about U.S. cyber-espionage against China, apparently in an attempt to curry favor with the Hong Kong and Beijing governments as they considered his requests for shelter. While Iran's invitation comes later than others, Tehran would surely be interested in hearing any details of U.S. spying programs against Iran.
Iran was not included in Snowden's earlier list of dozen-plus countries from which he'd requested asylum. But the country does have one potential benefit for Snowden: He could likely get there safely. Unlike Latin America, which would almost certainly require a flight over European countries that might attempt to bring him in, Iran is easily accessible from Moscow, perhaps by way of Turkmenistan or Azerbaijan, if not directly over the Caspian Sea.