I don't have a purebred dog, but I would not support buying where I could not meet the parent dogs and see that they were being cared for in the best possible conditions.
I don't like to buy battery farm eggs either or feed lot beef (aka Grain fed) and am a bit careful about what pork I get too. I can afford to choose to buy stuff that has been grown or made ethically, not using cruelty or slave labour.
The trouble with pet shops and "designer dog breeders" is that so very few of them are cruelty free - I don't know of any. You don't get to see the conditions the dogs live in - and the pet shop conditions are not good for puppies, no proper socialisation with other dogs or people.
Yes there is a demand and most people think it's fine to look up what they want on the net and go there without thinking about the baggage those puppies comes with.
Every day I walk at our local park and I see the health problems and some of the social problems those dogs have. At least all those dog owners have more of an idea of where to go next time they want a puppy.