The news is filled with talk about whether the US should honor its agreement to cover the cost of the THAAD missile defense system. However, I'm not clear about why the US agreed to pay for the system in the first place. Can anyone elucidate this?
The THAAD system is developed and owned by the United States so the THAAD system basically belongs to the US.
While other there may be other reasons for the deployment, the US's publicly stated purpose for the deployment in South Korea is to counter North Korea's growing threats and their use of ballistic missile and frequent nuclear tests.
The system also protects the US's own interests as North Korea has stated that they "has practiced attempts to hit U.S. military bases in Japan".
The acronym stands for Terminal High Altitude Area Defense. The ground-based missile defense system, which first came into development after the Persian Gulf War, is designed to shoot down short-, medium- and intermediate-range ballistic missiles during the terminal phase (i.e., when they are coming down).
On Tuesday, North Korean state media reported that the country has practiced attempts to hit U.S. military bases in Japan with a number of recently launched missiles. The number of missiles fired suggested that North Korea was training to see how quickly it could set up its extended-range missiles in a wartime setting.
If South Korea pays for it, it would mean that they are buying the system from the US. The US has mentioned that they are not willing to sell the system to South Korea:
The United States currently has six THAAD batteries worldwide and a former U.S. State Department official, who estimated the cost of each at about $1.2 billion, said Washington would not want to sell THAAD to Seoul.
"We want to retain THAAD in our arsenal, consistent with all other U.S. weapons systems deployed on the Korean peninsula. We own them. We retain them. We have the right to redeploy them," the official said, speaking on condition of anonymity.
Furthermore, the frontrunner in South Korea's presidential election, Moon Jae-in, said that Seoul will not pay for it and has even called for the deployment to be halted should the US request them to pay for it.
Kim Ki-jung, a top foreign policy adviser to Moon, said the suggestion that South Korea pay for the system was an "impossible option".
"Even if we purchase THAAD, its main operation would be in the hands of the United States," said Kim. "So purchasing it would be an impossible option."South Korea has never considered buying the system, partly because the cost involved was deemed prohibitive. Some liberal politicians have argued South Korea should develop its own indigenous missile defense system.
Thus, the system is the property of the US and since the US made the decision to deploy it in South Korea, they would cover the costs for the deployment and maintenance.
A major reason (beyond those Panda noted) are the threats of North Korea to launch a missile against the US directly. It's quite possible they could hit Hawaii or even the US West coast
“They want a long-range missile with a warhead on it,” said Jon Wolfsthal, a senior nonproliferation adviser in the Obama administration who is now at the Carnegie Endowment for International Peace. “We should be worried about the direction that things are going in.”