The ESTA VISA waiver which is the easiest form of entry to the USA for close allies like South Korea explicitly allows business activities. The idea is that it's meant to be easy to come to the USA for consultation, conferences, trade shows etc. The most basic VISA for countries like South Korea explicitly enables this.

If they came to the USA legally it's hard to understand how they didn't have the right to do things like consult on the build-out of the new factory.

If anyone wants to argue the ESTA doesn't allow this (despite the explicit wording that it does) you're basically saying no more international business conferences and no more business trips to USA offices without a very heavy weight multiyear immigration process.

Which is fine if you want to come out and suggest that. It'll kill what's left of Vegas, the airlines, conference centers, etc. It would also harm any international business in the USA but if you really really want to go around saying "ICE are right to detain the South Koreans here" I'm OK with that. I'll make sure that people from my home country understand that the USA is completely closed for business.