 The “while raising them for food when there are readily available alternatives” is crucial. It is sometimes permissible to cause animals pain: You painfully give your cat a shot, inoculating her, or painfully tug your dog’s collar, stopping him from attacking a toddler. The first premise is asserting that causing pain is impermissible in certain other situations. The “when there are readily available alternatives” is getting at the point that there are substitutes available. We could let the chickens be and eat rice and kale. The first premise asserts it is wrong to cause animals pain while raising them for food when there are readily available substitutes.