Already been mentioned, but for summary....
1. Adaptive cruise control will lower itself if you are following a slower car (e.g. if you set cruise to 75, it will slow to the speed of the car in front. Once that car is 'gone' and the road is clear, the car will accelerate to the desired speed).
2. The lane markings where this accident took place are in HORRIBLE condition, going off of the video footage that I've seen. It's easy to see how someone not actually paying attention could, in fact, follow this Tesla (almost) all the way into the barrier (which it seems someone else had actually done prior to the Tesla).
I will agree, though, that a radar upgrade beyond 1-D should be performed on all Autopilot-capable vehicles. If it can't tell that the thing that's not moving isn't something that it's going to safely pass by, I think that's a good indicator something is wrong. Right now, it's like getting the check engine light to go off by undoing the bulb - yes, it makes the problem go away, but at what cost?