Citation? I know from personal experience that this is not the case for non-US citizens.
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.