The distinction being that, when an airplane's autopilot disengages, the pilot's typically going to have a couple minutes to evaluate the situation and determine how to react appropriately, because airplanes typically don't just fall out of the sky. Tesla's "autopilot" gives you seconds to react if you wait until it disengages, if it knows to disengage at all.

Most Tesla purchasers aren't pilots; the meaning communicated to them but the branding "autopilot" isn't based on experience in aviation, it's based on popular culture.