What makes you confident they were acting illegally? Here's a US embassy:

> A B-1 visa may be granted to specialized workers going to the United States to install, service, or repair commercial or industrial equipment or machinery purchased from a company outside of the United States, or to train U.S. workers to perform such services.

https://es.usembassy.gov/visas/commercial-industrial-workers...

Many of those detained have been reported to be employed by Hyundai's equipment vendors. That would be consistent with activities of this nature.

It's probably a coin flip whether a different DHS staffer would agree, though. Interpretation of these rules has always been notoriously inconsistent, and probably explains the problem here.