Yeah I know I've heard that about Tibet too, has different rules to entering than the rest of China. Then I've heard there's even cities that ban westerns completely, but unsure if that's true.
Just a very odd concept to someone from North America. If you fly to Miami you are allowed to visit New York or Seattle all the way across the country. Same with Canada.
I know Mexico has some odd things like that too though, you can visit border towns or cruise ports but if you want to go so much more inland you need a Visitor Permit called a FMM, or tourist card some refer to it as. Read conflicting amount of miles, seems about 12 miles though in. So seems like if your cruise final departure was in Mexico and you flew back home you might need to worry about it, but a port of call at the beach seems like most wouldn't need it. However read also flights and cruises will also include it with your ticket, but if you were taking a road trip across Mexico you'd for sure need to get one yourself it seems. But seems more like a tax than anything, since not the same as a visa.