I bought my car after doing extensive research, so I knew I wasn't getting an autonomous car. I knew production autonomous cars are a decent ways away.
That being said, "AutoPilot" for the non-aviation crowd (me included) implies more than TACC + steer assist. And Tesla did more than steer assist, the car steers and keeps you centered.
I think it's a combination of calling it AutoPilot (for us non pilots) and the way Musk oversold it that people thought the car can do more than it can. "No hands on wheel", remember the rumors around that? And the no nag? (well they killed that last one, but still).
I have no problem with AP, actually I love it and use it on trips all the time. But like I said, I can see why some people might feel jipped.