That's how Google runs it cars at the moment.

Why can't the law just be, if you're behind the wheel of an autonomous car in possession of immediate over-ride, then you're exactly as liable as a normal driver?

That's the wrong question. You have to ask yourself about what the existing laws do. If one of Google autonomous cars that drives around crashes into another car I would guess that a court would see Google as being responsible in addition to seeing the driver as responsible.

The law exploits this illusion of control people have all the time. Why not here?

That's no good line of argumentation when you want to convince a politician to pass a law in your favor.

if you're behind the wheel of an autonomous car in possession of immediate over-ride, then you're exactly as liable as a normal driver?

Economically it makes sense to have cars that work like Taxi's where the human driver doesn't have a way of immediate over-riding the system.