>From an insurance "at fault" perspective it's basically always going to be the fault of the following driver.
It is a quick simple rule that avoids dealing with he said she said situations in the days before dashcams and the inaccuracy of this approach would roughly balance out across the insurers. Sure it screws the people who aren't actually at fault but they can't (or couldn't, before dashcams) prove that anyway. This kind of thing will probably go away in the future because everybody will just have dashcams.
It is a quick simple rule that avoids dealing with he said she said situations in the days before dashcams and the inaccuracy of this approach would roughly balance out across the insurers. Sure it screws the people who aren't actually at fault but they can't (or couldn't, before dashcams) prove that anyway. This kind of thing will probably go away in the future because everybody will just have dashcams.