He can't do illegal things. It's perfectly within his rights to deny someone service beforehand.

Since Tesla vehicles have a lot of functionality that is basically SaaS (firmware updates, functionality updates, remote start/unlock via application, etc), if Tesla were to say "You gave us a bad review, we're not going to provide you any updates to your vehicle and you are not allowed to use any of our authorized service centers for repairs", what law would they be breaking?

Check your contract. When you buy a Tesla, what OTAs are you buying?

...and this is why I will never buy a Tesla, no matter how cool they might be.