No, the borderline would be that there is a victim. While animals are not afforded "rights" they can be clearly victims of inhumane treatment. Hence we have such laws. While the penalties are weaker than those of cruelty to humans, they are afforded some protections under our laws. Even in a libertarian society they likely would be afforded the same. One could even look upon it as preventative, as those who are into animal cruelty often expand their cruelties later.
Very true.