They do expect you to change that posture if Autopilot does something you think is dangerous, though. Wandering out of lane uninitiated (lane change indication, for example) and accelerating towards a barrier would likely count as one of those situations.

I've yet to see an incident so far where a human driver, properly engaged with the driving experience and road conditions as they should be when in charge of a vehicle, couldn't have taken action and prevented the accident. There were 7 seconds between the beginning of the maneuver and impact; Human reaction time (including taking action) is typically 0.5 seconds. That leaves 6.5 seconds to correct steering or apply the brake, bearing in mind that Autopilot will always relinquish control in either situation.

Not apologising for Tesla, they need to sort out this edge case, but that's exactly what it was and exactly why the driver is supposed to remain engaged.