This doesn’t have anything to do with taxes or registration, etc. - it has to do with aesthetics. SJ thought a license plate detracted from the design of the car and therefore was not needed. That’s it.
Your source of this knowledge?
Bingo - Jobs didn’t want an ugly “logo” on the car, so he left the plates off. It’s the most logical explanation.
A little bit of OCD can go a long way.