Aren't most pet dogs vaccinated for rabies today - or they are supposed to be?
You're absolutely right, dogs from the pound, a vet, a pet store, or other commercial venture usually include the vaccination in the adoption fee...however...
The dog you get from a friend's litter may not ever see a vet. Someone may get a dog from PetSmart, get it vaccinated once, then never again. Dogs from an unwanted litter that are dumped, IF they survive (and a lot do), never see a vaccination.
Wild animals are ALWAYS a rabies threat.
Better to be safe than sorry. The penalty for guessing wrong is severe.