In the wild, sick or injured offspring attract predators. It's not mean or cruel, it's a survival technique. We as humans do not have this issue - if we have a sickly baby, there is health insurance, and we have doctors. No predators will come after us. If we have allowed out pet to have a litter, and the cat/dog abandons a pup or kitten, then we have a moral responsibility to get vet care for that pup or kitten, and spay the female so that she can no longer breed. That does not apply to the wild. Any animal that has a litter provides food for animals higher up on the food chain. Cats eat mice, and foxes, coyotes, and birds of prey will eat kittens. Lions and wolves eat baby deer. The idea is that enough mice or kittens survive to carry on with the law of Nature. We find it repulsive, but the animals do it to survive.