I'm pretty sure this is not always the case.
Yeah, but it is 99.9% of the time, and that's what really matters. If you asked someone, "would you rather have that random person or that random dog killed," most people would pick the dog (unless they happen to be crazy animal lovers).
Again, just because not everyone would rather have a dog die over a human doesn't mean that your point is valid. There's always an exception to everything (well, almost everything). A human life is still more valuable than a dog's.