Digging a little deeper into your state's requirements I found this in several places.
The State of California requires that all water heaters be strapped to resist motion during an earthquake. This may be accomplished by installing an over the counter “water heater restraint” kit approved by the Office of State Architect (O.S.A.).
and...
Some manufacturers make listed straps for water heater installations. Listed bracing straps may be used in lieu of the above methods when installed per the manufacturer's installation instructions.
(My bold)
None specifically mention that OTC "strap kits" need to be metal.