I look at it this way- Our heroes are the offspring of the Nephalem or the Nephalems children, they are therefor 25%/12.5% demon/angel and 75%/87.5% human. They are dominantly human and can't be considered demons or angels anymore. Therefor, humans they are.
You wouldn't call a Chihuahua a wolf, even though it's technically of wolf ancestry would you?