EDIT: In the US this is covered by the Fair Labor Standards Act, which is extremely simple when defining what is considered on-the-job work. If it is mandatory, work-related, and for the benefit of the company, them it is on-the-job work and you should be paid for the time. So congratulations; you've likely participated in wage theft by onboarding people who aren't being paid for their time.

Except for the other reply that starts "you are right. you cannot onboard a new job before you leave your old one"??? They may go on to say that accepting an offer isn't onboarding but since I never tried to argue that it was, that's kind of irrelevant.

Lots of people don't know their rights or their obligations. Wage theft is the #1 from of theft in the US by a lot. Coordinating with an IT department for onboarding without getting paid for it is straight up wage theft and being taken advantage of. Doing so while still employed by another company is moonlighting under most contracts.

People do shit like that all the time. Doesn't make it right. Doesn't make it safe.