For some people, having a dog they know will kill is a selling point. They are misguided in keeping the animal for a pet, but they believe this animal will kill to protect them.
More often than not, the animal kills their own pets before it moves to human prey. These owners are the ones who keep the dogs in a frenzied state, which makes them more prone to attack.
Unfortunately, we have laws against putting the owners to sleep, but the dogs should be put down. The owners should be prosecuted, not just for the destruction of property, but for the treatment of the animal who dies because of their twisted idea of what a pet is.