Don't know how old you are but I'm pretty old.One of my first passports (issued early/mid 1970s) said on one of the first pages that it was not valid for travel to several countries.I think Vietnam might have been one of them but I *know* North Korea was.
IIRC some court ruled years ago that US citizens couldn't be forbidden by the government to travel to a particular country.
I’m not that old. Born during the Reagan Administration.
If that is the case, I can understand that the government can’t physically stop you from traveling to that country; however, at that point the risk is completely your own and the US government will not provide assistance.