You weren't necessarily acting unlawfully, and neither were the Koreans. From reports, they seemed to mostly have an ESTA or B-1 visa. This doesn't allow "work", but it does allow many business activities. For example:

> 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...

Most other countries have similar policies. The line between those permitted activities and "work" is notoriously blurry. This often results in unexpectedly denied visas, and sometimes results in nasty letters or fines. It almost never results in this kind of show of physical force, even in unfree countries.