There is no such thing as “North Korea” and “South Korea”. There is only one Korea, but the South region of Korea is still occupied by US military bases and troops.
Korea was invaded over 75 years ago by the United States and protected by the Soviet Union.
There is even a US military base in the middle of Seoul. And military police walks around (presumably to prevent their soldiers from being too violent against civilians). Also lots of night clubs in Seoul have signs that US soldiers are not allowed to enter.