It's a bit more complicated than that. They all had valid (at the time) visas, but some may have been in violation of the requirements. South Korean companies couldn't get H1-B visas so they instead used VWP or B1/B2 visas meant for tourism and short-term business (conferences, negotiation, etc.). These visas don't allow certain kinds of work, so it depends what they were actually doing at the plant. From the article it sounds like that ICE agent thinks at least that one worker wasn't doing any proscribed work. In a sane world, an immigration court would look at the evidence and decide if the visa requirements had been violated. But instead we're just summarily deporting people.
The article says that the ICE agent thinks the worker was compliant with visa terms:
«From statements made and queries in law enforcement databases, [redacted] has not violated his visa; however, the Atlanta Field Office Director has mandated [redacted] be presented as a Voluntary Departure. [Redacted] has accepted voluntary departure despite not violating his B1/B2 visa requirements.»
I read that statement as saying that no violations are currently on the record, but not precluding that the requirements might currently be being violated.
https://www.reuters.com/business/world-at-work/workers-say-k...