Originally Posted by 335BBS
I believe it is law. Just like rear ending someone.
Firstly, insurance companies don't care about the traffic law and are not required to care about the traffic law. Their resolutions are based on the principle of minimizing their losses.
Secondly, there's no law that would state that rear-ending someone automatically put the rear car at fault. There used to be such law in some (or most) US states a while back, but it only resulted in a wave of insurance fraud crimes. Crooks in several cars operating in concert would orchestrate accidents, making people to rear-end them (which is rather easy to do for a group of skilled drivers).
Thirdly, in practice rear-enders often use the "unsafe lane change" defense, claiming that the front car made a dangerous lane change right in front of them. If they really stick to it (fake witnesses etc.), the front car's insurance company often abandons the case ("couldn't prove the liability").