But the carmarker is surely within their rights to refuse to continue servicing your car, or declare that any attempt at modifying the electronics/software potentially makes it unroadworthy.
Having said that, I don't entirely understand why Tesla don't keep the software unloaded from the vehicles until the user chooses to purchase the add-on features: compared to everything else the software does, that's not exactly a particularly difficult engineering challenge.
That would likely be highly illegal of them, per the Magnuson-Moss Warranty Act. Unless they could prove that the process of you enabling the confiscated features caused something else to break, they're still on the hook for it.
Having said that, I don't entirely understand why Tesla don't keep the software unloaded from the vehicles until the user chooses to purchase the add-on features: compared to everything else the software does, that's not exactly a particularly difficult engineering challenge.