Not sure where you got that information, but it contradicts everything I've ever heard.
Cuba's been for a long time the only country the US won't allow its citizens to visit (head, meet desk); North Korea is, so far as I know, the only country that actively prevents US citizens from visiting, although that may be a misrepresentation of the facts on the ground.
US tourists visit North Korea; Vice magazine did an interesting doco on their experience. Tourists are chaperoned and experience cartoonish levels of propaganda, but they're not blocked.
If you actually watch that video, Vice had to bribe a Russian border official to gain entry to the country. It's generally known as the only means of getting in with a reasonably high odds of success.
The typical experience for Americans who have applied for visas to visit North Korea is a very long wait, with only gaining approval (sometimes after years of applications), only to have the approval revoked sometimes as late as hours before their flights depart. Some people report having had their travel visas revoked three or four times before finally being granted entrance.
"Not blocked" doesn't seem like the right term for whatever that is.
I didn't recall their issues getting in, but watching it again, there were no Russians involved. They ended up going to the North Korean consulate in Shenyang, China. No other embassies would help them get in.
US citizens can visit Iran—at Iranian discretion—with a state minder the same way US citizens can travel to North Korea. You can't travel freely and you can't travel without personal scrutiny ('extreme vetting') from Iranian officials.
TL;DR: Visa applications take longer (but still usually <30 days) and you must be accompanied by a guide at all times, but other than that you're free to travel around.
It's worth noting that this is considerably looser than North Korea, where you not only have a guide, but can only go to the places that have been agreed in advance and your guides permit you to go to.
