I think it is a CA emissions regulation that limits the sale of aftermarket catalytic converters. This from the CARB website:

"..Aftermarket catalytic converters must be exempted from California's anti-tampering laws in order to be legally sold and installed in the state. If an aftermarket catalytic converter is shown to be durable and meets vehicle emission control requirements, it is granted an exemption Executive Order (EO) that allows it to be installed on specific emission controlled vehicles...."