(Not a lawyer) Generally, at least in the US, when you own a physical object, you can do whatever you want with it that isn't explicitly illegal. This is at least partially represented in the first sale doctrine. For a car, you have a literal entitlement document showing that you own it. What you're not entitled to is any help from the manufacturer to do so, except that repairs can't void a warranty if those repairs didn't cause the problem being warrantied. With regard to batteries, there's no reason someone who owns a Tesla shouldn't be able to replace or upgrade the battery they own on the car they own.

Sure, but what does that have to do with what happened here? No one tried to hack the battery management system. The company modified it as a goodwill gesture (and to ensure that it didn't cause a PR disaster by functioning as intended.)