I'm a pro-rescue dog-lady and am against puppy mills... And I'll tell you why.
We made "dog." We domesticated them, took them into our homes, made them dependent on us. We have made them man's best friend and their lives are enriched by human interaction and companionship just as much as we are by theirs. For us to neglect them, exploit them, or abuse them is the worst act of betrayal we could possibly commit. They love us with every fiber of their being.
I often joke that I'm not for the basic right of pet ownership, but of "a human for every dog"... Because every dog should have a human to call their best friend. We created them as we know them today, and we sure as hell should accommodate them.