Developers can select the country app stores their apps are available in. It wasn't relevant for my app but if an app can't really be used in the other country why risk support requests and bad reviews when you might actually want to launch a functional version there one day.
Why? To use the examples of the grandparent comment: there are always overseas Japanese who want to cook, or tourists who want to use a local navigation app. I can only come up with legal reasons, or bandwidth preservation maybe.