An ESTA is not a visa. The USA publicly announces that Korean citizens have the right to enter the USA for up to 90 days for business or tourism. As we're seeing though, the real rule is "don't look foreign".
This statement is just completely disconnected from the reality of the diversity of the American work force.
Even under ESTA, you can do some such activities - call'em "job" - in the US. On behalf of your non-US employer.
The devil may be in the details, but the assertion these workers were "likely" (or even just "potentially) doing a _US job_ (subject to a Visum qualifying for _US employment_) is definitely misplaced.