I think that their argument relies on assumptions something like these: - One should avoid causing unnecessary harm to another conscious being - Many non-huma

I think that their argument relies on assumptions something like these: - One should avoid causing unnecessary harm to another conscious being - Many non-human animals are conscious beings - It is practical to survive on nutrients not derived from the harm of conscious beings If you accept those three statements, then you should be uncomfortable with the idea of consuming nutrients derived from the harm of something conscious. If you choose to do it anyhow, you'll experience cognitive dissonance (where the definitions of "harm" and "consciousness" remain constant throughout the described scenario).