I think the point is that if the vehicle's software can't figure out how to cope with something as simple as a cone on its hood, it's woefully unprepared for real driving conditions and a liability to everyone
It might seem trivial, but it's not too hard to imagine a traffic cone on the windshield actually being a fairly dangerous situation if the car simply drove off, for instance if could roll over the top of the car and impact another vehicle behind it.
And that's even assuming that the traffic cone can be identified as 100% being a normal traffic cone, and not something else (say, a heavy object suspended in some meansm, in which case driving into it would damage the car) or a person or animal that happens to look like a traffic cone.
So by demonstrating that the cars stop under unexpected situations they are showing that the cars successfully identify a potentially dangerous situation and prioritize safety over navigation.
Also, I doubt it's a software problem: the cone is blocking a pretty substantial field of view. No amount of software can fix an occluded sensor. They might as well be spray painting over the cameras.