As it is from puppy mills, so too from kitty mills, the kittens are at a much greater risk of contracting a disease, or having a genetic condition from inbreeding. The end result is large medical expenses and quite possibly early death of what becomes your beloved pet, should you purchase from this type of place.
Very often, what amounts to a kitty mill is more accurately described as a backyard breeder. Whether it's someone who wants their children to witness the miracle of birth, someone who just hasn't bothered to get their cats spayed and neutered, or someone who figures they can make a few extra dollars by selling kittens, the resulting "products" are the same as those from kitty mills - sickly, unsocialized kittens. Whether these kittens are sold in a pet store, an online website or through classified ads, unsuspecting buyers purchase what they think is a purebred, special cat.
The following is something we wish everyone read - and it's a good idea to spread it around. Too many people are not aware, do not realize the inherent dangers of not neutering their pets, or in thinking it would be a good idea to "have kittens".