I think it all lies in the definition of "negligence" as a legal term, not the everyday meaning. The Wikipedia article (
https://en.wikipedia.org/wiki/Negligence) makes interesting reading because this term has slightly different meanings even across English speaking countries. Not to mention the non-English speaking ones. A person from our aviation authority during one of the many courses I have taken once explained, that there can be no accident without some degree of negligence (acts of god like being hit by an asteroid and force majeure apart). If everybody involved applies 100% care to erveything he does there can be no accident. So for an accident to happen, a certain degree of lack of care - or negligence - is required. We are all humans and few of us can be 100% diligent 100% of the time. We make mistakes, which, in legal terms, translates to: We are negligent. To cover us, and the people we might harm, we take out an insurance. Which is mandatory in many countries for operating aircraft. And insurers have to pay out for accidents caused by negligence, this is exactly what they are there for. They don't have to pay if the court rules, that "gross negligence" was the cause of the accident. But there are very very few cases in aviation where a court has come to that conclusion and in this case of the ferris wheel that term was not even mentioned.