score:2
I think there are two different lists at play here. One is the list of countries with which Korea has a visa-waiver-agreement. The US is not on this list.
Then there's another list of countries citizens of which can enter and visit Korea without a visa. The US is on this list. Citizens of the US are allowed visa-free entry for up to 90 days.
See for example the list at the Ministry of foreign affairs or the list you're linking to. HiKorea has the same information:
Nationals of the following countries are allowed up to 30 days of visa-free sojourn for tourism or visitation Exceptions: Canada is allowed up to 6 months, and United States, Australia, Hong Kong, Slovenia, and Japan are allowed up to 90 days.
...