As you can see, instead of extending the DMZ westward into the sea, the U.S. line runs northward, limiting North Korea’s sea access. The line was drawn this way for two reasons: First, when the fighting stopped, South Korean forces were in control of the islands off the North Korean coast and the U.S. wanted to secure their position. Second, control over those islands enhanced the ability of U.S. forces to monitor and maintain military pressure on North Korea.
North Korea never accepted the NLL. It argued for an alternative border, illustrated by line B, the red West Sea Military Demarcation Line (MDL). Acknowledging the reality of Southern forces on the islands off its coast, North Korea sought recognition for a sea border that went around the islands but otherwise divided the sea by extending the DMZ line.
The critical point here is that the South Korean and U.S. promoted NLL is not recognized by international law; it has no legal standing. Don’t take my word for it. The following is from Bloomberg News:
“Then-Secretary of State Henry Kissinger wrote in a 1975 classified cable that the unilaterally drawn Northern Limit Line was ‘clearly contrary to international law.’ Two years before, the American ambassador said in another cable that many nations would view South Korea and its U.S. ally as ‘in the wrong’ if clashes occurred in disputed areas along the boundary. …