"Autopilot" is understood to mean "the plane flies itself" by the general public. Here's one human pilot who's ticked off by it:
http://www.askthepilot.com/questionanswers/automation-myths/
http://www.askthepilot.com/questionanswers/automation-myths/
Those constrains could be constant like a certain heading or a direction (not the same thing by the way), a series of vectors or a single vector (like a flight-path or an approach to a runway) or constraints that depend on dynamic variables like controlling the altitude and speed according to the weight of the aircraft or automatically avoiding traffic.
Of course the pilot must be standing by because a) autopilots do not keep track of everything (and aviation control interfaces are quite complex) and b) sensors can and do malfunction.
Just because people mistakenly believe something doesn't make it right. The general public once thought the Earth was flat, and how did that turn out?
Tesla promotes it as autopilot. Words matter.
If Tesla itself selectively denies that the common understanding is the meaning they intend, then they appear to be trying to have it both ways: cashing in on the PR and cachet that the common meaning confers, while disclaiming the responsibility that goes along with owners actually believing it.
The unfortunate truth is, if I gave my mom a car with the autopilot feature, she'd end up eventually trusting it and not care about the implementation details that make Tesla's system not an automated driving feature. That's scary and dangerous to market to consumers.