Because the original parent comment is using an analogy to aviation terms, and depending on a more than cursory understanding of the limits of an aircraft autopilot. I am willing to make a monetary bet most members of the population do not understand the precise limitations of an aircraft autopilot, though I admit I have not conducted a poll to verify this. I also think Tesla likely named it this to differentiate it from the competitions systems, and in my eyes it is pretty obvious what distinction they were aiming for most people to make.
It's the same thing as putting "fat free" on the Twizzlers. It's technically true which means they can get away with it. Except that that probably kills more people than "autopilot" ever will.
There are a hundred ways to say that something won't cause you to gain weight. They chose the one that sounds like it means that even though it doesn't, because it also has a different meaning which is technically true.