On the contrary, California emission standards held diesel passenger vehicles to the same standards as gasoline/petrol passenger vehicles. Whereas in Europe, diesel emission standards were much more lax compared to their petrol counterparts.
I suppose one could say that being more concerned with the more directly harmful and smog forming emissions(NOx and PM) over CO2 would constitute 'bias' against diesels. But, conversely, there's a rather compelling argument that the more lax European diesel regulations spurred diesel adoption and protect their domestic manufacturers.