...these puppy mills, which happen to be completely legal, fulfill that demand...
What you say is true. Until recently though, what was legal was the kind of factory methods that when used used to raise companions yielded serious health and behavior issues, so many simply boycotted the part of the system they could. Missouri was a notoriously popular place in the country for this kind of farm because the laws were so lax.
Laws were recently changed and many puppy mills closed down, but there will still be some who do not care for factory methods producing animals.