My question is why does anybody have to be liable at all? Most normal people would consider this just to be a freak accident.
Sure, there's learning points that can be taken from it to prevent similar incidents - e.g. erecting a fetch around the field (why didn't you suggest that the field owner be liable) as it can be reasonably foreseen the situation of a ball escaping and being a nuisance to someone else (maybe it just startles someone on the road, maybe it causes a car crash, whatever), or legislating bars or plastic film on the barber's window, etc.
But here nobody seemed to act in any way negligently, nor was there any law or guidance that they failed to follow. It was just the result of lots of normal things happening that normally have no negative consequences and it's so unlikely to happen again that there's nothing useful to be gained by trying to put the blame on someone. It was just an accident.