In the TV series Being Human, the only-human-blood thing for vampires is justified by them mentioning that it has to be human, and it has to be fresh. The lead vampire, who is on the wagon, was recommended the blood banks at the hospital, but packaged blood doesn't sate the thirst, and nor does animal blood. However, the rest of the vampires have the traditional us-and-them attitude that means they just don't care enough to try alternatives.

In the same series, the werewolf is trained away from human meat by a scent trail from a butcher's joint. The killing-humans behaviour of werewolves is therefore justified by the fact that humans are generally the closest living (and therefore strongest-smelling) meat around when the werewolf transforms.

I just thought of a great potential rename or redirect to this trope: Humans Are Delicious. Tee hee hee.