They do though! They can easily disable features like Autopilot, Autopark, Summon, etc fleetwide in a snap if they wanted to.
Technically, this is true for any product which has a pathway for software updates. Arguably this includes nearly all models of car on sale today, which can have their software updated — even if the process is more cumbersome.
The key difference is that a Tesla vehicle can conveniently perform comprehensive whole-of-car updates while it's sitting in your garage, whereas many other cars can only have some systems updated during a service visit.
Regardless, I don't see how software updates being convenient could serve as the defining criteria for a manufacturer having "ownership over the vehicles they sell".
Somewhat related question: do Teslas have a EULA or other agreement on purchase? And if so, does it include an arbitration clause?