Except you can't buy a car from another state and register it in California if it doesn't have a sticker saying it passes California emissions standards.
The car manufacturers didn't used to put the 50 state or CA emissions stickers on all of the cars. I'm assuming they had to pay extra for the sticker. This led to problems when people started buying cars on the internet. If you bought a car considered "new" from out of state (less than 7K miles and I think less than 10 years old) you would not be allowed to register it in California until you've had it in CA for at least 2 years. No test you can take to get around this. It had to have the sticker or sit in your garage for 2 years. When I bought my car from out of state I had trouble getting a loan because of this. The credit union had run into this problem on a previous loan so they just wouldn't deal with out of state purchases anymore.