Wow! The question was whether to treat butcher birds for a sickness not the prophylactic or growth promotional levels of antibiotics.
A sick animal should be treated, not doing so violates just about all accepted standards of animal care.
Actually, I think he asked what I would do. I said I would cull it. And here is why...
The growth rate of meat birds slows down considerably when they are sick. By the time you get them well, you have already wasted good feed on a bird that isn't going to reach its full weight potential, anyway. What is more, the symptoms sound like it could be MG, and MG can't be cured. You can only treat the symptoms and make them recede. In the end, you still have a sick bird that could then (once it is "cured") be reintroduced to the flock and infect more birds, which would then also cut into their feed-to-meat ratio and stunt their growth (if it doesn't kill them, first) and thus ruin the entire crop. What is more, even if you get the MG into remission and don't reintroduce the birds to the rest of the flock (which increases your workload to take care of just a few birds that you are going to lose money on because they aren't growing, anyway), you are still selling sick birds to your customers. And THAT is not only irresponsible management, it is illegal, as well.
Now, you could cull a bird and have an expensive necropsy done on it, to make sure it isn't infected with MG and then treat accordingly, which would take time (and you would still be feeding a scrawny meat bird) but to me, it is just simpler, cheaper, and more humane to just kill the darn thing, burn it, and get on with the job of raising the rest of your healthy, growing, meat bird flock.