That's not obvious at all.
It's not even necessarily true.
In some states the the automated driving system, when engaged, is legally the operator.
In some states the owner is legally responsible for any legal violations of the vehicle, even on an L4 or L5 vehicle. (Even if he's not in the car BTW).
In some states the owner (or his agent) is legally responsible for SOME things in an accident (like reporting if they hit an unattended vehicle) and on the hook legally if they fail to, though not necessarily the accident itself.
In some states they appear to just entirely YOLO it by including language like
"LIABILITY FOR A CRASH INVOLVING AN AUTOMATED DRIVING
SYSTEM DRIVING A MOTOR VEHICLE THAT IS NOT UNDER HUMAN CONTROL
IS DETERMINED IN ACCORDANCE WITH APPLICABLE STATE LAW, FEDERAL
LAW, OR COMMON LAW. "
And then not actually including any state law to govern the situation at all liability-wise.
My approach was to simply it all down to just a person in an L4/L5 vehicle, and what responsibility they have when they are the driver and what responsibility they have as the passenger while it does the driving.
No responsibility or liability can be assigned to an individual in an L4/L5 vehicle while it's autonomous driving mode. This is assuming the individual can't mess with settings that would impact the legality of the drive, and they don't mess with the controls while its driving.
Now sure if they are the owner there are responsibilities that come with that position, but that's different than simply being in the vehicle.
That isn't to say that what you brought up isn't potentially hugely important. Obviously if I as the owner had responsibility for an Tesla FSD accident I would think twice about signing up for the Tesla fleet network.