Generally speaking, US businesses are not allowed to do business with/in Iran (there are some exceptions). In practice this means that American accommodation finders can not (or will not) offer accommodation inside Iran as this would run the risk of them breaking US law and getting fined immensely for it. Yes, with the current nuclear deal being a near-reality, this is likely to change soon.
How messed up this is, is shown with that even many businesses that are not American but do business with businesses in America choose not also do business in Iran, as that can result in them being banned from doing business with the US (or be fined accordingly).
Stay on top of things with information directly from the US treasury:
An article on what the nuclear deal could result in, business wise:
A (year old) article on that the US penalises foreign companies and individuals for doing business with/in Iran:
A recent article of the German Commerzbank being fined heavily, in the US, for obfuscating financial transactions with Iranian entities:
The Couchsurfing website has users in Iran.