Its all about corporate liability being huge compared to the negligence of a random schmuck.I think your missing the point.
All of these cars will have sensors that will pickup on this stuff outside the car.
where as a human can be easily distracted with a phone etc, a hot girl on the corner etc.
It has sensors, great, and if these sensors malfunction and provide skewed data, or heck perhaps are simply dirty? My GPS sure as hell isn't flawless, it tells me to go down roads that don't exist anymore all the time. And we know autopilots for aircraft that don't even have to worry about anything being on the runway can make mistakes, which is why pilots are paid so much. And sometimes the mistakes aren't software, but something goes wrong with the hardware, and a computer may make a completely unreasonable conclusion on the course of action.
Now, yes, a human can go "full retard" under those circumstances too and be held liable for negligence, but who is going to go after some $32K a year single-mom teacher? Not worht a lawyers time, but Toyota has already proven that if there is even a HINT that blame can be placed on a major manufacturer you will have a team of the best lawyers on the planet working around the clock to collect their millions of dollars.