In addition to exacerbating backyard breeders and adding to the overpopulation of uncared for pets, animals should never ever ever be an impulse purchase, as pet store kitten and puppy cages end up often being. and many parents, who were pressured into buying a kitten (or puppy) for a wailing child, may feel embarrassment when they realize that they actually can’t care for the kitten properly, so instead of bringing it back to the pet store they dump it. Kittens, puppies, in addition to things like ducklings which should never be sold in pet stores, are suffering daily due to this. There are so many homeless animals already, this is simply adding to the problem. Shelters are underfunded and overwhelmed, dogs are nauseatingly overbred in puppy mills, thousands of kittens and cats on the streets are susceptible to every kind of violence and illness. 
STOP THE CYCLE.