Enterprise, like banks and stuff, want/need to have official support from a vendor usually for these kind of things. Even for Linux there's Red Hat for example. Just patching security holes themselves hardly cuts it.Koreans can use other browsers for general surfing, but when they go to an eCommerce website, they'll get a message telling them they need to use IE. That poses even more problems for Apple users, for which IE isn't available.This isn't really much different from making a government app, that only runs on a certain brand of telephone.
Any time a government places that many restrictions on the way their citizens can behave it causes all kinds of suspicions about the government. It is highly likely that the government is tracking the internet usage and computer usage of their citizens by using a bug placed in the program.
Requiring that South Koreans must use Internet Explorer for online shopping and banking creates a lot of suspicion toward government officials. No country should be able to mandate which internet browser its citizens must use. This law gives the appearance that the government is trying to use Internet Explorer to spy on its citizens.
Forcing South Korean citizens to use Internet Explorer for any of their shopping or banking needs is a clear sign that their Government does not trust them with their web browsing. This is a sign of state surveillance and paranoia which cannot failed to be noticed by the citizenry themselves.