I'm of the opinion that if one owns an autonomous vehicle, regardless of software modification or not (which should be allowed), then one is fully responsible for it's actions. If one doesn't trust the software provided by the manufacturer, don't buy/use it. Once one chooses to buy it and operate it, then it's that person.

Possible exceptions would be in the case that, after purchase, the manufacturer pushes a software update that meaningfully changes the behavior in such a way that it causes issues. In that case, both A) the manufacturer should be responsible and B) the owner should have the option to get some kind of compensation.