There is fundamental difference between the US troops staying in S Korea and Japan. You will see S Korean president giving medals to US troops served in Korean war but you won't see Japanese prime minister giving medals to US troops for fighting Japanese during WW2. US troops are staying in S Korea not just for S Korean defense but also defense of the US from N Korean and Chinese ICBM because radars in S Korea will detect them 30 min before radar stations in Alaska giving enough time to intercept them. Also the US bases in Japan are in threat by Russian missiles but Russia will not attack US bases in S Korea because both S Korea and Russia has good relationship. Technically Japan and Russia is still in war because they never signed peace treaty after WW2 because Japanese want back lost islands after they started WW2 in Asia.