How is fault proven for non-AV collisions? There is no ground truth in those cases either; the best case is that there's dashcam video, which may be sufficient to give an approximate idea of what happened and who is at fault.
If Tesla ever achieves a pure-vision Robotaxi, its eight cameras should give a very good idea of what happened and who is more or less at fault in a collision or incident, even if it's not understood why the Robotaxi acted as it did. (If Robotaxi makes a mistake, it doesn't matter why it made the mistake; it's still Robotaxi's fault, even if the sensor suite was incapable of detecting and avoiding the accident, in which case it's Tesla's fault for designing an inadequate sensor suite.) It's ultimately a subjective human decision how to assess and assign fault. I don't think having ground-truth LiDAR data adds much value here. If the fault isn't obvious (to humans) from the camera feeds, LiDAR data is not going to make it any more or less obvious, I think. The raw LiDAR point cloud is still several steps removed from the semantic processing required for object classification and so forth, so there is an inevitable degree of neural-network opacity that is practically impossible to untangle.