This claim is variously stated:
It's ok to eat food from non-factory farmed sources or "humanely slaughtered" animals.
"Our animals have all good days and one bad day".
People will turn around and claim that animals have no rights or that somehow "human vs not human" is a meaningful distinction. But this would seem to contradict this claim. Saying that "factory farming is wrong" is admitting that animals have some moral value and that factory farming is
For instance, in the cited discussion, Dillahunty went on to admit that he ate meat at a restaurant the night before. And anytime sourcing is brought up, it's about antibiotics and meat quality, such as in the Intelligence Squared debate. To date, no serious popular carnist ethicist or socio-political commentator has said that they boycott all meat they object to on ethical grounds on a source-by-source basis. So those who propose this argument probably don't believe it anyways or at least don't follow it. Does anyone even know of someone that eats vegan in every circumstance except for local meat?
We don't accept this line of reasoning in any other circumstance. We wouldn't accept a serial killer's actions as ethical because he targeted specifically well-to-do attractive young girls to give them "one bad day". It also seems unlikely that someone making this claim would accept babysitting for their own children from someone who, for instance, gave random songbirds the "humane slaughter" treatment for their own entertainment.
Imagine any set of the following set of moral axioms that are commonly used and what they might have to say about killing an animal after giving it a "good life":
There is, of course, one notable exception: "Might makes right".
By killing animals, you're still hurting other people, starving people of needed food, and destroying the environment. The "good days" for that animal weren't so good for the others carnism hurt along the way, either.