To your second point, nothing on those vehicles says "warning, no human in control" so I'm not sure how obvious it really is.
You absolutely can; it's what you're supposed to do if a toddler runs out in front of the car, for example. Barring malfunctioning brake lights, if you rear end a suddenly stopping vehicle, you were at fault for following at an unsafe distance.
Most legislation about this (example: https://codes.findlaw.com/ca/vehicle-code/veh-sect-22109/) says stuff like "when there is opportunity to give the signal". Screech brakes, then signal.
But it's pretty hard to enforce just for the reasons you mentioned. "I saw something on the road" is a fairly bullet proof defence which nobody can counter.
Kid in the street is a reason to stop suddenly