I doubt it's an act of carelessness, anyone with that kind of a balance would at least know how and where to input the appropriate fees.
You assume that all mistakes come from the lack of knowledge or experience. But there's a wide variety of other reasons that can force someone to make a mistake, like being tired, sleep-deprived, distracted, etc. I don't see anything strange about a supposedly experienced a wealthy user making such mistake.
If anything, it shows how dangerous Bitcoin can be, because it allows such things to happen. So this should be a reminder that with Bitcoin you are your own bank, meaning you are responsible for checking your transactions and no one else.