They want to avoid thinking about non-engineering things like liability for as long as possible and focus on the engineering problem. It's a valid approach.

As for the "who has liability" question it's simple: the driver does. Or at least it's intended to be simple in the absence of lawyers twisting the wording around. Regular, non self driving cars have basic driving assists. For example, cruise control will happily ram your car into the back of another one. Adaptive cruise still has a chance of doing that, plus it has the additional risk of causing an accident by randomly slamming on the brakes for no reason at all. For all these failure modes the liability is understood to lie with the driver: it is up to them to be alert enough to override the automation when it malfunctions. However, if you do a long distance drive with and without cruise control you'll quickly admit that it has value, even with all the imperfections. Same for Comma.