As the CFHS writes, «The production of whole litters of kittens who have no future homes must be curbed.» (catsandbirds.ca)
Feral kittens are kept in a minimum of pairs (usually siblings) as one kitten on it's own would be really frightened, however keeping whole litters or larger groups of kittens together generally delays taming. (celiahammond.org)
While a reputable breeder who occasionally stumbles on these specimens may sell these runts with a strict spay and neuter contract, so that these specimens are not allowed to reproduce, an unethical breeder may instead purposely breed their runt with another runt (often closely related by inbreeding) in hopes of producing whole litters or smaller than average dogs. (pethelpful.com)