So if they used state specific codes, what happens when the vehicle owner moves to another state? The vehicle owner has to replace their ECU? What if someone is driving in another state and their car breaks down? Also, that would require them to have another model specifically for Massachusetts. I think it's unlikely that a car company would go to that much effort just to spite the other 49 states. Besides, just imagine the PR nightmare it could become. Some TV news station (or news website) will run a story that says "ACME Automotive won't let your mechanic fix your car! More at 11!"
Real Users hate Real Programmers.