The idea that I advanced is that animals do not contribute to society, therefore they deserve no "rights" in the sense that humans know "rights". Lefty is my dog, and I will protect him. This means that if some sicko decides to kill him, he better not stick around.
Another way to say this is that animals do not live in a society and cannot be bound by a moral code. For example, it is perfectly acceptable for a bear to kill a rabbit. Bears are known to eat a range of things, including small mammals, fish, berries, etc. The bear doesn't need to eat a rabbit to survive---it could just as well eat fish or berries. The bear, however, offers no protection to the rabbit. And if we passed a law protecting rabbits, bears wouldn't care.