So Korea and Japan have been bickering for decades over a pile of God-forsaken rocks in the middle of the ocean. If you ask a Korean, he'll tell you that Dokdo is located in the East Sea and was an integral part of Korean territory for centuries before the Japanese occupation. If you ask the right kind of Japanese person, she'll say Takeshima is located in the Sea of Japan, and that Korea thinks it's theirs. If you ask almost anyone else, though, they'll probably stare at you and blink uncomprehendingly, or ask (rightfully) why anyone cares.
Well, apparently the Seoul Metro cares: they've installed these propaganda displays at various points throughout the city. If I were a more cynical person, I might say that they are positioned specifically to catch foreign eyes (the message is in both English and Japanese).
Seems like an odd way to greet a population of tourists who spend a ton of cash in your town.