What good is a safety system that you cannot depend on? To allow a vehicle to steer on your behalf requires, by definition, some abdication of control to a system that Tesla says is not dependable. That's the reality of it.
It is still in its infancy phase so it is reasonable to depend on the human skill and not the automation's reliability for collision avoidance.
The owner's manual does not explain why they allow an imperfect Autopilot on the road but here's the background:
Owners need to realize that there are 2 prevalent approaches to lead to the goal of driverless.
1) Google/Waymo does not trust human skill to take over the automation system when needed so it will only release its system to consumers when human intervention will no longer be required.
The advantage is: No one will die because of the system while the project is still being worked on.
The disadvantage is: the project may take a long time and the public might not be able to get hold of the technology possibly by 2021 the earliest.
2) Tesla believes if you are good enough to drive, then you are good enough to use Autopilot even when it is not perfect and requires human interventions at times to avoid collisions.
The advantage is: The public gets a hold of the technology even when it is not dependable.
Before I got AP2, I was driving with a basic cruise. It wasn't perfect. I had to intervene lots of times to prevent a collision. But to choose a car without a simple cruise and a car with one, I will always choose the one with one.
Now that I am using AP2, I still have to intervene from time to time to prevent a collision. But if I have an option to drive one without or one with it, I will always choose the one with AP2 even I know that I cannot depend on it but it is still very useful to me.
The disadvantage is: People can die if they depend on the system to reliably avoid collision.
In summary, although the manual does not go into the background but it is up to owners to choose which approach to pay for (manual, simple cruise, Autopilot, or wait for future Google/Waymo driverless).