"i think it's very situational. if i left my children in the care of a sitter and when i returned home my house was in shambles and the sitter was sitting on the couch snapping bubble gum and listening to her walkman while my kids were hanging from the light fixtures and she said: "by the way," you owe me $50 for your kids drawing on my leather jacket," my inclination wouldn't be to pull out my wallet."
This is part and parcel of why I disagree with your level of perceived responsibility. Look at your own example, and consider how far out of line you had to go to prove your point. I agree that someone who's that disconnected and neglectful wouldn't deserve reimbursement, but there's nothing in the original situation that indicates that the nanny was anywhere near this bad, and these edge cases keep getting rolled out to say that not every single situation falls to the parents. I agree that there's never any 100% guarantee, but I'm trying to hew closely to the situation that was actually presented, and I find that the vast majority of reasonable assumptions about what happened put the responsibility for replacing the iPad squarely on Herman's parents. I can easily create scenarios that put the blame on the nanny (she decides she wants an upgraded iPad, so she purposefully leaves it on the table and lets Herman destroy it, for example) but considering that Herman's parents trust her enough to let her take him to her home to watch, I have to assume that she's not grossly incompetent or malicious.