Tesla knew drivers were going to turn on Autopilot and then stop paying attentions, so they did their research

“It appears that over the course of researching and developing Autopilot, Tesla considered the possibility that drivers could misuse the system in a variety of ways, including those identified above — i.e., through mode confusion, distracted driving, and use of the system outside preferred environments and conditions,” the report from the National Highway Traffic Safety Administration reads. Tesla engineers considered that drivers might fail to pay attention, fall asleep, or become incapacitated while using Autopilot. So they ran their vehicles through different scenarios to evaluate how Autopilot would handle each.