I don't know for certain. However, I recently read about the Freedoms of the Air, which define international airline abilities to move about in various ways; a

I don't know for certain. However, I recently read about the Freedoms of the Air, which define international airline abilities to move about in various ways; and some of that includes where you can stop temporarily (fuel planes, etc) and pick up passengers. I wonder if some of this has to do with a cost structure that comes out of that? I'm also totally willing to believe that it's not a fully rational system and is more about demand/profit for certain routes. https://en.wikipedia.org/wiki/Freedoms_of_the_air https://en.wikipedia.org/wiki/Freedoms_of_the_air