NPR had a good story about the large dog breeds and the people who breed them and buy them when this story first broke last year. It basically showed that with dog breeders, as with every other profession, there will always be a minority of people who are quite willing to sell a product that is dangerous to an individual who doesn't care or actually revels in the danger of the product. The story is here - you can click and then select "listen to segment" to hear a RealAudio clip:

Big Dogs
Demand has soared for the type of dog that fatally attacked a California woman. Vincent Duffy of member station WKSU reports that breeders are resisting this new clientele who seek big, vicious dogs. (4:30)