They weren't "starting a job" in the US. Their (foreign) employer sent them onto an assignment abroad (to the US) to do their job. That's the difference here.
Even under ESTA, you can do some such activities - call'em "job" - in the US. On behalf of your non-US employer.
The devil may be in the details, but the assertion these workers were "likely" (or even just "potentially) doing a _US job_ (subject to a Visum qualifying for _US employment_) is definitely misplaced.
Correct, they already had the right to conduct business for 90 days even without an ESTA, because that's what the law says. An ESTA is only needed to physically enter the country.