Am I allowed to crap on the carpet?

One argument used by meat-eaters to justify killing animals for food is the "But animals do it" argument. It sounds reasonable: since animals slaughter other animals for their food, why shouldn't humans slaughter animals for our food? Aside from the fact that people mostly eat animals that don't eat other animals (cows, chickens), this argument is shaky at its very core.

Let's assume that animals are amoral beings (for if we call animals moral beings, added to what we already know about the intelligence and emotions of animals, we must further question whether they should be eaten). It does seem rather absurd for humans to get our morality from amoral beings. It conflates the behavior of beings that have a choice about their behavior with beings that have no choice in their behavior (indeed, some animals are biologically required to eat meat, while humans are not). Are proponents of this defense of meat-eating willing to apply this same logic to any other issues? Because if we're going to judge one activity based on the behavior of amoral beings, should we judge all our activities on the behavior of amoral beings? I'm not entirely uncomfortable with this idea (as Dostoevsky frames it, that "everything is permitted"), but I doubt very much the proponents of the "But animals do it" argument are comfortable rejecting all human ethics. Do they want humans to behave as animals in other aspects of life? Do they want to claim that humans are really no better than the animals we're willing to eat? Do they want to claim a superior species would be justified in eating humans? Do they want to reject the premise that humans are rational beings capable of choosing our actions?

I think this smacks of defensive justification for a desired behavior, not an objectively reasoned out position.