> Can't people then just register the car quickly out of state and then re-register it locally so it looks like "a move" legally and not a sale?
Paying twice the taxes on a new car is not a cheap thing.
The states cannot prevent you from importing a car, but they can make it a huge pain in the butt. Multiple days of having to take off work, getting the run around and misinformation from bureaucrats, spending hundreds of dollars on fees and inspections to get to what is free if bought in-state. They can be very nasty about it.
(don't ask me how I know. This is based on personal experience)
How unpleasant it gets depends heavily on the state and how much of a vendetta key politicians will be willing to go on when they have a corporation thumbing their nose at their laws.
It's not just registration. Most people buy new cars on credit. Which means they don't have the title. You'll have to provide a out of state title, which you don't actually own, to the state. You'll need insurance that is accepted by the state and your creditor. You'll have to make sure that the insurance agency can communicate this correctly to the state and provide them the right information, or it may be multiple trips to government buildings until it gets sorted out.
And it's not like the state you are buying from is going to be all that happy about losing out of probably will amount to several hundred dollars in sales taxes.
Each of these things is just another thing to go wrong. Another day waiting in lines and getting bad information from people who would rather spit in your face then talk to you.
All of this stuff is very doable, of course. Probably take just a week or two of screwing around at the most, etc. And I am sure that Telsa would help streamline the process as much as possible, but they would still lose customers because of it. Getting dragged through the mud for what really is just a luxury car is nobody's idea of a good time.