Right, if that were the case, the major carmakers wouldn't bother making separate models for the Canadian or Mexican markets, either (and unique ones at that not available in the US because of emissions, diesel, crash worthiness, etc). Most are made side by side with vehicles for the American market in the Canadian and Mexican plants--and most are penned in the corporate design offices next to their American counterparts.
There are literally millions of cab, engine, drivetrain, interior, and transmission combinations a pickup comes in, and they somehow manage to make all of them instead of making one version and hoping people will run with it.
There's really no reason they couldn't have separate drivetrain and ECU combinations for Commiefornia and the rest of the country, unless you're also saying making the same car with two or three engine choices is so damn hard (Camry, Accord, etc), or a Mexican, Canadian, and American spec one in the same factory or with nearly the same blueprints. It's not cost savings, it's just plain laziness. It would be cheaper to streamline all model lines into one popular engine and transmission choice the way they have with the 4Runners, yet you don't see many manufacturers doing that.
In truth it's led to aerodynamic add ons and engine management crap, transmission shift patterns, oil blends, etc. that most of the country does not need (the majority of the car market is still outside California) and the ideas are so brilliant, political administrations latch on to them and make CARB the law of the land--which is slowly killing off things like diesels, body on frame SUVs, big V8 sedans, etc.
One day when you're driving your unibody crossover SUV with turbocharged four cylinder gas-hybrid engine and low rolling resistance tires think of profitability, politicians, corporate boards and CARB and smile.