Since there is nothing in the California Plumbing Code (modified UPC) requiring an earthquake valve, this must be a municipal requirement. Try calling the plumbing
inspection department and asking for a supervisor. Explain your situation and ask his opinion. If he agrees with your plumber, have him put it in writing. Then contact the field inspector and, as nicely as possible, explain to him what you have done and ask him to take it up with his boss if he has a problem with it. On the other hand, if the supervisor agrees with his field guy, you're SOL.