Since you asked, I've worked for an environmental regulatory agency in California for the last 15 yrs. You can probably guess which one. Needless to say, I correspond with my EPA colleagues on emission-related assignments regularly.
That's irrelevant though. As a gearhead in California, I was simply pointing out the blatant misinformation in your post when you said "If your car is 1973 or older, you do not need to add cats to your car except in California." That's absolutely untrue. As @72bluNblu pointed out, anything '75 and older in CA is smog exempt and is never required to undergo any kind of emissions-related inspection. I'm amazed that CA isn't as stringent as some other states with respect to older vehicle emission inspections. I'm not complaining though - hopefully other states follow suit. I'd imagine this bin makes up a tiny fraction of the overall mobile source emissions inventory in the US. I can't imagine why it would make sense administering and enforcing these rules on older vehicles. Well, I guess I can, but then this thread would be moved.