Normally I turn the steering wheel when I want to turn my car. If you sold me a car and told me it had technology to make turns automatically without my input then I might let go of the wheel instead of turning it, something I would never have done otherwise. If I then don't turn and slam straight into a wall, am I at fault for trusting what I was sold to be true?
If the driver has heard that their Tesla is capable of autonomous driving, and therefore trusts it to drive itself, there may be a fair argument that Tesla shares in that blame. If it's a completely unreasonable belief (like me believing my 1998 Toyota is capable of self driving) then that argument falls apart. But if Tesla has promoted their self driving feature as being fully functional, used confusing descriptions like "Full Self-Driving", etc, it might become a pretty reasonable argument.