> If the "industrial scale" is the main point, that looks like a solid argument - except why focus on animals? There's also tree plantations, mines, cities, and anything else that isn't sitting in the jungle doing craftwork.

I think the "factory farming" is the point. The manner of which animals are treated throughout their lifetime, and the manner in which they are dispatched, would make most people squirm. What's worse is that animals are shot and mostly just disposed of to keep crops from being eaten, and in many places it's illegal to sell meat from these or other wild caught animals. Often this is justified by a need to control disease or parasites, but more likely it's to protect the livestock industry. And of course, livestock will be crammed as close as possible to each other to 'increase productivity', and this poses a significant biohazard, as we've seen recently with the avian flu outbreak (and prior bird flu outbreaks, swine flu, mad cow disease etc. etc.) putting to question this justification of public health.