In these cases, humans are being degraded to animals. And from a literary standpoint, somewhere in the cycle of the protagonist's story he/she must degrade himself before a final confrontation. PETA raised the status of chickens and placed them with humans.
A less severe version of the 'Holocaust On Your Plate' would be an issult you say to someone you didn't like. 'You smell like a pig'. 'You've got the brains of a rabbit'. A majority of people do not like being put the the level of an animal.
A lot of this boils down to where you place animals and humans. While I love animals (heck I live with four of them) I think humans are superior. We are smarter, have a mental capacity for abstract thought (thank you Clooney), and can reason. Compassion is one of those abstract thoughts, and I have no objection showing it to animals. But all the same, I also have no problem consuming one.
