H1-B for non-profit - Work on OPT and then H1B?

Hey, I am currently on my post-completion OPT and I have got a job offer from a non-profit organization. The HR mentioned that they would give me an offer letter and once I accept it, they will file for my H1B and only if the H1B is approved I can start working with them saying that is a regulation in California. I wanted to clarify if there is really a regulation like this because I was under the impression that I can start working on my OPT while the H1b is being processed. Can you suggest some negotiation points as to how I can convince the HR about this regulation?

Technically they can hire you on OPT and you can work. But, it is their preference probably…
You can tell them that you can work on OPT and show them the proofs that OPT allows you to work in the same area as a student and then once you get H1B, then they can update the I-9 compliance with the H1B details… You can talk to the immigration department who handles your H1B at the employer and get their inputs told to the HR, so that they can allow you to work. Do update what they say…