Earlier quoted context omitted.
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. Anyway, http://uncorneredmarket.com/american-travel-iran/…
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.
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.