When you consider that many people are not aware of puppy mills, and you think about it, it is easy to see. If someone goes to this place and sees the horrible conditions, they will, understandably feel pity and shock over the poor puppy and many would want to buy the puppy to "rescue" it. Being unaware of this huge, miserable industry out there in mass producing puppies, they just don't know they are supporting something like that. It is easy to see how they would think the place they have seen is an isolated case, and they have done something good for this one puppy, having NO idea they are dooming so many thousands of other puppies.
Just like many people are unaware of human trafficking (I even had a guy in one of my classes argue that there is no such thing, it is all made up and that if people are being exploited they are letting it be done willingly), many more are unaware of the problem of puppy mills.