https://en.wikipedia.org/wiki/Criminal_negligence
In your case, the person was being reckless. I don't think gaining access to hardware you own can be considered reckless, regardless of the unlikely circumstances that may result. Your death-by-rooting example is entirely hypothetical and has never happened.
Just like drone accidents, each year we get closer and closer to an actual kill as their number increases (https://youtu.be/MvF49R_ZX5E). One day heavier drones will be required to run only certified software if flown over crowds, and rooting those drones will raise the same question.
It has never happened? Who cares, what does that matter?
By your logic, if the manufacturer puts out software that is responsible for a death, would you throw the software developer who wrote in jail? That's silly.
All this stuff is limited to civil liability.
If I root my phone and accidentally brick it, nobody here would say, "Gosh, that's terrible, it's all Motorola's fault!" They would say, correctly, that if blame is to be assigned, it'd all be mine.
I think it's even clearer with something where even small errors could be fatal. Anybody rooting a car is betting both their life and those of the people they encounter on their technical skill. It might be an excellent bet, but they should be entirely aware that they're betting.
Yes?
So is every guy who pops his trunk and starts working on the car? And people have been doing that since the very beginning. This isn't anything new.
There are three new things here.
One is that automotive mechanical systems are relatively well understood. If you are a mechanic working on a car, it was literally designed for that. The manufacturer makes manuals explaining how to do it well, and they sell extra parts that are meant to be swapped in. There is a century or so of evolved practice an understanding here. This is not true with drive-by-wire software, which is certainly novel, and is frequently terrible.
The second is that mechanical stuff is generally linear and easy to understand. If I change a lightbulb, I have a pretty good understanding of what that will affect and what it won't. Software is highly non-linear. A novice working on, e.g., a device driver, can have effects they will find extremely surprising.
The third is that in software we have a long culture of consequence-free tinkering. We're used to being able to power-cycle something, to restore from backup, to just re-install the OS. Any software person is going to have a lot of habits and biases that become extremely dangerous when working with life-critical systems.