Lots of ways to interpret this. Personally, I place zero blame on the victim and feel she should of course be compensated. Petting a strange dog is not the best idea but it does not make you responsible for the outcome imo.....especially when the dog is in a store.
The primary blame imo is the dog's owner. Her dog. her responsibility. Period.
Don't bring it unleashed in public and there will be no issue. If the dog really had zero history of aggression, still too bad. Her dog. Her responsibility, period. The best child in the world that breaks a neighbor's window only once, or punches someone in the face only once. Parent's kid. Parent's responsibility, parent should pay damages, period.
The secondary blame is on the store because of their ridiculous and ambiguous policy. Without a doubt that will change really fast and imo it should. Animals have no place in stores unless they are service animals. People have allergies, biting potential - etc. Of course this is all just opinion but i feel pretty strongly about it.