summery:First, they must submit detailed proof in the form of laboratory dyno test that document their part does not have an adverse effect on exhaust emissions. These tests are very expensive and must conform with specified test procedures. CARB then reviews the data and may or may not ask for additional information and/or testing. If the product meets CARB's criteria, CARB issues an "executive order" (EO) number (also called an "exemption" number) certifying that the part is in compliance with the applicable clean air rules
http://ca.autos.yahoo.com/maintain/emission_control_answer3.html
basically due to the fact that it costs way too much; most manufacturers dont even bother with california unless its is a big ticket item that they know they can make their money back on. wonder why throttle body and aic kits cost so much?