This appears to be a way that the Korean government may try and hit back at U.S. negotiators trying to work out a new USFK cost sharing agreement:

Members of the ruling Democratic Party of Korea (DPK) Thursday floated the idea of including South Korea’s indirect financial contributions for the U.S. Forces Korea (USFK) as “leverage” if Washington keeps asking Seoul to pay “excessive” costs.
Korea Times
“If Washington keeps making absurd demands, South Korea will seriously reconsider the adjustment of indirect costs created by the USFK apart from defense cost-sharing,” Rep. Cho Jeong-sik, chief policymaker of the DPK, said during a party meeting, Thursday.
Underlining that U.S. President Donald Trump’s current rumored demand for Seoul to pay $5 billion for U.S. forces here is “excessive” and not written in the U.S.―South Korea Status of Forces Agreement (SOFA) or the Special Measures Agreement (SMA), he added the U.S.’ demands for Korea to pay more in additional categories such as rotation of U.S. forces and off-peninsula drills are irrelevant to the defense of the Korean Peninsula.
“Korea is making large contributions in many categories directly and indirectly. For example, Korea is letting the USFK use harbors, railways and land for free. It costs 3.5 trillion won as of 2015. Seoul can [recalculate the indirect contributions] and use them as leverage,” another DPK lawmaker who is familiar with the SMA negotiations said during a phone interview with The Korea Times.
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