California has a big push to get residence to register their cars in this state. They know full well that there is a strong incentive to register your car in another, cheaper, state. The police are well computerized, and will dog you until you comply.
I remember your earlier thread where you were asking advice on how to get around these laws. Some members gave you some advice, which now in retrospect was faulty.
Some people get away with it for a while, but others get busted early on.
I believe the officer was under orders from his superiors to run a tight neighborhood, and used your lack of a plate to start the proceedure.
If you are going to live in Calif. you will need to accept the enhanced ordinances. (When in Rome)...
You will probably need to get a Calif. drivers license.
Another option is to get a Mexican license, and register your car there. That is almost like a free pass. They won't even give you a parking ticket, as they have learned from experience that they will not be paid. No emissions issues, cheap registration, and almost like embassy plates for teflon immunity.