This is a good idea to not only discourage travel to North Korea, but also put a process in place to detect a intelligence collection threat that could be coming to the U.S.:
Any South Korean who visited North Korea after March 2011 will no longer be able to visit the United States visa-free, according to the Ministry of Foreign Affairs.
The U.S. government said it is putting new restrictions on its Visa Waiver Program, or VWP, for citizens of 38 countries including South Korea who have visited Iran, Iraq, Sudan, Syria, Libya, Yemen, Somalia or North Korea.
Under the VWP, South Korean citizens are eligible to visit the United States visa-free for up to 90 days as long as they register through the Electronic System for Travel Authorization at least three days in advance.
This is no longer an option for South Koreans – or citizens of the other 37 countries – who have visited the North. While they will be allowed to visit the U.S., they will have to obtain a visa from a U.S. consular office.
Joong Ang Ilbo
You can read more at the link.