Why do people say, "Blame the owner for a dog's bad behavior?" How so, when a dog is unpredictable?

Part of the skill of dealing with dogs is learning how they behave. You watch them until you learn how they behave and are able to read the signs that come before certain behaviours. You anticipate what they might do. Not everyone is equally good at this, due to differences in experience and abilities. Most of those attacks "out of the blue" were not that at all. People simply failed to understand, observe, or react to the dog's body language properly or in time. Cats are just as capable of doing damage to a human, but since they're a lot smaller than the average dog, and a lot more likely to just try to leave if there's a problem, it doesn't happen often.