> Usually when a company needs to send someone to another country to do training or some work for a week or two they just go as tourist
What?
You simply say you're traveling on business, you absolutely do not lie and say you're a tourist.
If you have ever had your visa denied, you are no longer eligible for VWP tourist visa as government finds it high risk of doing something incompatible with your visa.
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.
B1/B2 visa allows some construction work: https://www.cbp.gov/sites/default/files/documents/B-1%20perm...
In particular, worker training and equipment installation/setup. This seems to be exactly the case there.
In other guidance, "construction" is specifically excluded from the permission to "install equipment". That's not a very clear distinction, especially for large industrial machines, though I do see their general intent, to permit work requiring equipment-specific skills but exclude work that a locally-hired employee could do about as easily.
https://travel.state.gov/content/dam/visas/BusinessVisa%20Pu...
I don't think this greatly changes your general point. It's likely that many of those detained were in fact lawfully present.
But what is "construction"? If it means "building walls", sure.
But what about assembling and installing the battery presses? Or maybe setting up the electrical connections for them?
It is a genuinely ambiguous area, and I won't be surprised if Korean companies tried to push it a bit too far. This still doesn't excuse the ICE behavior a bit. They could have just revoked the visas and asked employees to leave.
It's also quite clear that the Korean companies were not flying engineers here to save on wages.
I have a friend in an amateur band, they wanted to do a US tour so they applied for a performers visa, paid thousands of dollars and got the application denied, so now he can't even go as a tourist. <shrug>