>According to the file written by an agent with Homeland Security Investigations (HSI), an agency within Ice, the man entered the US “with a valid B1/B2 visa”, which allows for some business-related activities and tourism. He was at the Hyundai factory as a contractor with a South Korean company.

I did some research on B1/B2 Visa's it looks like the work allowed is rather restricted. https://news.ycombinator.com/item?id=45207369