Indeed. Why would it be considered cruel? I do know that animals raised in more humane or organic methods do not require the same use of hormones and antibiotics, since they are generally kept in cleaner pens, fed healthier foods and given room to roam and thrive. Now in a more industrial feed-lot style livestock raising, then yes, antibiotics are necessary as that particular method has high rates of sickness transfer. In that scenario, I could see how not using antibiotics could be considered cruel, since the risk of disease or illness transfer is so much higher.
But other than that? I certainly don't think for a second that animals raised in clean, healthy and organic/humane methods do not need a continuous stream of antibiotics, Their diet, healthy lifestyle and all the other wonderful benefits of being raised humanely/organically can usually combat any immunity upset without medical interference.
My Gramps used to raise pigs organically, and they handled any upsets beautifully. He did take them to the vet if he was concerned with anything, but they never required antibiotics, and if there was something wrong, under the guidance of a vet, he would treat them naturally. Never once lost a pig by doing it like that.
But that's just my two cents on the subject.