Every offer letter I've ever received has language to the effect that it's contingent on a number of things, including a background check.
Companies usually fuss about it, but I insist, and among quite a few orgs over the past 25 years I've never had one pass because I refused to sign prior to background check.
Lesson learned.
I think one key thing in that California law is the notion of the “conditional job offer.” This law doesn’t effect the dynamics of the offer letters we were discussing at all. If you make an offer and there are still conditions pending, you’re obligated to let the candidate know. I cannot imagine CA is doing that differently.
I get your point, though, that doing the criminal portion of the background check after extending the offer is now not just normal, but mandatory, in CA.