All or nothing, eh? Figures.
Yes, I'm all about personal responsibility. How about exercising a little bit of it, and placing responsibility where it belongs - on the person who let the dog out. Is there previous history that shows the person who was watching the dog wasn't responsible enough? Or is this the first time they've done something like this? Unless you have factual knowledge that the owner was aware that the person who was watching the dog is irresponsible, I'd say the owner made a good effort at responsibility. Being the owner, they maintain some responsibility. However, SO DOES THE PERSON WHO WAS SUPPOSED TO BE WATCHING THE DOG.
Lumping 100% of the blame on the owner is the lazy way out of this situation. The world isn't black and white, slick.