It doesn't need to be intentionally programmed to cause an accident. The far more likely scenario is that it simply doesn't take into account what's behind them. It causes accidents by unintentional negligence, simply because nobody bothered to add a check.

Human drivers are taught to avoid scenarios like that. During drivers' lessons you will be taught to check your mirror before braking, and you will be taught to activate your alarm lights when there's a sudden stop on a highway to alert the drivers after you. Basic self-preservation makes it pretty clear that you don't want a car crashing into your rear.

Waymo doesn't need to care. If the other vehicle is considered at fault, their insurance is going to pay for your damages. It's at worst a minor inconvenience, so why bother spending engineering time on writing code to avoid it?