Overclocking can cause harm (worse battery life, excessive heat). That potential for harm is a plausible reason to void the warranty.
Tesla is just voiding the warranty as a punishment for opting out of their software-gated features. Those features aren't harmful to activate.
How is that different from increasing the maximum acceleration of a car? Worse battery life and excessive heat. It is entirely possible that people think Tesla is greedy instead of Tesla choosing one trade off at the expense of another in software.
Tesla's more expensive models often share the same battery and motor but they tend to push the hardware closer to its limits which also results in higher wear and tear. High performance cars usually need more frequent maintenance. It is entirely possible that warranty on the cheaper model is longer than on the higher end model and by installing the upgrade Tesla can no longer guarantee that longer warranty or any warranty at all.
Is it only possible or is it a fact? If it is only possible and not a fact then it is a fact that warranties are the same.
It's more as if you bought a laptop, and in order to overclock it, you had to void the warranty. (which is, of course, entirely the case for almost every laptop on the market).