A major concern with pet stores and the reason why puppies and kittens may not be sold in certain stores anymore is largely due to puppy mills. Puppy mills are commercial dog breeding businesses that breed dogs primarily for profit, often with little regard for animal welfare.
In pet shops, animals must be viewed as commodities in order for the store to realize a profit. This means that, in order to cut costs, animals are too often kept in inadequate conditions and denied needed veterinary care.
The cost of providing veterinary care for an animal kept in a pet shop can easily exceed the animal's commercial value — meaning that animals may be left to suffer or even die from untreated illnesses or injuries.