From an editorial in The Korea Times:
Subcontractors of two mobile carriers sold location information of 200,000 customers. The theft shows a deep hole in the protection of location information, and is a grave infringement of the privacy of subscribers.
Police arrested engineers working for SK Telecom and KT for stealing and selling location information, which the telecom companies claim took place without their knowledge.
Investigators say private detectives and marketing companies bought the data. Private detectives misused the information for chasing unfaithful husbands, wives and lovers at the request of spouses and partners. This would provide evidence for divorce litigation or breakdown of spousal ties. Marketing firms can also analyze the consumption patterns of the subscribers.
Read more on The Korea Times.
Update: Web Pro News provides some additional details.