The Tesla Model S isn't a self driving car. Tesla Autopilot is a beta version of a driver assist feature that makes it exceedingly clear in prompts before you can use it that you're expected to keep both hands on the wheel and pay attention.
Autopilot is closer to a glorified cruise control that a full autonomous vehicle like the Google cars.
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Friends of the family say he was using his laptop at the time. He could easily have been watching Harry Potter.
Also important to note that a witness who was driving 85 MPH at the time said the Tesla passed like she was sitting still. He was clearly going wayyy too damn fast for that road.