My understanding is that the US's refusal to join on a full international ban on land mines is based on worries about the Demilitarized Zone between North and South Korea. The US feels that land mines are needed to keep North Korea from invading South Korea.
The US has said that it will not sign the ban until it can figure out some effective way to protect South Korea or until North Korea is no longer a threat. The idea here is that North Korea has so many troops and the border is so close to South Korea's capitol of Seoul. Without land mines, the US feels, it would be impossible to prevent an invasion of South Korea with the number of troops that the US and South Korea have.
You can see this logic in the following quote from the jmu.edu link
While landmines continue to maim and kill large numbers of civilians around the world, they can, through deterrence, prevent war, in particular by protecting American and South Korean forces and civilians from being attacked by North Korea, thereby avoiding thousands of potential casualties.
Mines have only limited effect in preventing military incursion. Even during wartime, less than half the casualties of mines are military, as war ends or moves away from a mined area the civilian casualties rise radically. This treaty was denied during a periods when the Legislative branch of government was held by the party with the greatest financial ties to military contractors.
The US for a long time was actively involved in campaigning to put a stop to the use of landmines during war as they cause more civilian injuries after the war is over. A global consensus was finally reached on this matter in 1997, a treaty to stop the use of landmines was framed and it has been signed by all the nations except 37 which include the US.
Why the US refuses to stop the use of landmines is unclear as every reason that has been used to ratify the continuation of their use has be countered with a rational argument. And the US is familiar with the damage that landmines cause, over 30% of injuries caused to the US armed forces in Iraq can be attributed to landmines.
I guess is it just a lack of political will that has led to the continued use of landmines.