Comment by anigbrowl

It's easy to enter the US on business under the visa waiver program, but I think there are a lot of restrictions on what sort of business activities are acceptable. Thus it's fine to do market research or pitch your product to potential investors or distributors, but if you do work and collect a paycheck inside the US (even if it's at the office or factory of a Korean company) you need a proper work visa.

This case is additionally complicated as some of the construction work was being done by Latino workers who (like many construction workers) may not have had valid immigration paperwork.