It doesn’t really matter what’s behind you. If there is a danger in front of you and you don’t brake, it’s your fault, if you hit the brakes and someone rear ends you, it’s their fault for not keeping a safe braking distance.
You’d never design your system to intentionally be at fault.
Safe breaking distance is 2 seconds of travel between two and the car in front of you. In my experience almost no one ever follows this guidance and therefore yes, the rear-ender is always at fault.
I’m imagining some kind of future system where self driving cars are constantly scanning for tailgating, and when it happens, the number plate is logged and the data sold to insurance companies so they can adjust for high risk drivers.
Modern traffic radars have enough spatial and velocity resolution to apply a tailgating measurement. There are existing speed camera systems that can detect and ticket vehicles for tailgating, after a manual review of video and telemetry data (often just a quick glance and mouse click from a police officer).
There are relatively few jurisdictions that have a concrete quantitative definition of tailgating. So it's not yet widespread, but law enforcement certainly has the option available now.
The future will be that California will pass a law that mandates that newly manufuctured cars cannot tailgate at all, just like the one pending to beep at you when speeding.
In all serousiness, eventually, driving will be removed from humans.
You’d never design your system to intentionally be at fault.