This seems like a category error. We don't typically assign moral agency to animals the same way we do to humans. No one is saying "one biological creature eating another biological animal is unethical." Some people are saying it is unethical for humans, who we typically do believe have moral agency, to eat other animals. Just as no one would say it is unethical for a snake to kill someone with its venom, but we would say it is unethical for a human inject someone with snake venom in order to kill them.