Snowden re-tweeted a post by Nima Fatemi, an Iranian independent security researcher, who published a letter from Amazon saying that the company had "identified your account during a recent check for countries and regions where we are not authorized to provide services." Fatemi noted that the company's cloud services were exempted from US sanctions by law.
Amazon said it had "a policy to comply with government sanctions and export control regulations."
Iran is currently under the wide-ranging US sanctions that were restored after Washington pulled out from the landmark 2015 nuclear deal.