Short question: why do you say that one who adheres to determinism considers individuals to be genetic blank slates ? (Disclaimer: I know very little about genetics) It seems like if certain things will "inevitably" make us react in a certain way, there must be a genetic component to these rules. 

Several factors make me confident regarding the importance of this choice : the sheer scale and intensity of the suffering involved, the lower cost of helping nonhuman individuals in farms compared to humans, and the comparative small size of the aniimal welfare / advocacy movement giving $100m a potentially more important long-term impact.