I guess I don't understand how.

> A navigation mechanism, as on an aircraft, that automatically maintains a preset course.

Applies here. As far as I can tell the system did do exactly that. But the details of the actual event are unclear (I've heard parked car but also just hitting the back of another car?)

It’s an emergent technology. The onus is on Tesla to be crystal clear about capabilities, and consistently so. People might quite reasonably infer that something which is labeled as “auto-“ or “automatic” doesn’t require supervision.

Such as automobile?

Automobiles are self-propelled, not self-navigating. They don't rely on horses to pull them.

automobile: self-propelled (self moving)

autopilot: self-piloting (self navigating)

https://www.ahdictionary.com/word/search.html?q=automobile

Indeed, since you do not need to externally supervise the drivetrain in a car.

It didnt maintain the course. It kept going at a t intersection and ploughed into another car