Also they're interested in minimising risk in the future and providing recommendations to achieve that. It may well be the case that in a strict legal sense Tesla has no responsibility here, but from the NTSB's point of view that's irrelevant—the question is whether there's anything Tesla could do to minimise the risk of future incidents like this.
If Tesla relies so heavily on lane markers, someone may very well perform "a hit" on one or more Tesla owners by painting some adversarial lane lines. They could even tuned to the spectral response of the Tesla sensor pack.
