If feeding an hebivore the flesh of its own species, other animals and sawdust, all likely contaminated with fecal matter isn't mistreatment, I don't know what is. I understand there are some serious differences between what you feed an animal, and flogging it against a wall. But what is at issue is that all of these problems are systemic. When we don't care if an animal is tortured before death, or has its beak and claws removed, and lives in a cage all its life where it shits onto the animals in the cage below it, we start not to care what we feed them, what kind of medicine, steroids or antibiotics they are given, if it is sick or not etc.
When people don't care, systemic problems go un-fixed. I don't mean to imply that smashing a chicken will make you sick, but that if people don't care, these kinds of problems will arrise.