Why do you assume his/her parents didn't know any better? Some people really are selfish and malicious. You're going out of your way to assume the best when you don't actually have any reason to believe that's true.
You can apply the same line of thinking to humans. Everyone is a victim of their circumatances and experiences.
And while I understand that "everyone is innocent at heart" might be true to only some degree both with humans and dogs (nature vs nurture, etc), you can't really tell by the end result.