I imagine this won't apply if the insurer just leaves the state.
Yep. These are terms to operate as an insurance company in the state. If you don't want to do that, the rules have no bearing on you.
Which effectively means that anybody in a less risky area of California is just subsidizing those who live in the risky areas. Premia across the board will increase as a result.
Typical California redistribution...but this is from the bottom to the top.