If I can just take a shot at helping to answer your question Power Lunger.....
I think the reason that people may go a breeder instead of a shelter to get a pure bred dog is that they want the experience of raising a puppy.... Many people choose a pure breed based on color, personality, etc. and the chances of a shelter having that specific puppy I'm sure is slim to none... I can't imagine too many breeders dropping off a litter of puppies at a shelter although I very well could be wrong I don't work at a shelter..... I also have a bleeding heart for shelter animals... but like everyone else has said I believe the problem is because of irresponsable owners not people who buy from breeders...