Some puppy mills make deals with people they have sold bitches to, that if those people will breed to their studs, and surrender the puppies to the kennel at 7 or 8 weeks old, they can get the female free. That way the mill doesn't have to have facilities for caring for pregnant females and raising puppies, or cover the cost of caring for the pregnant female and their litters, but they still have puppies to sell.
Read between the lines here (I took out the name of the kennel offering this "service" because I don't want the casual reader here to think this is a great idea and buy a bitch from this mill):
OWN ________ FINEST FEMALE PUPPIES AT NO COST!
________ observes the time-honored tradition of reserving its finest female pups as breeders to maintain the quality of its internationally recognized lines. These pups are selectively entered into the ________ breeding program based on their outstanding conformation which meets or exceeds breed standards, as well as their stable temperaments, winning personalities, health and physical stamina.
Under our no-cost breeding agreement, each pup is home-raised by a selected family and returned to be bred at the appropriate time to one of ________'s champion studs. When the pups reach 7 weeks of age, the family delivers the litter to ________ and the "mom" is theirs to keep, free and clear. This practice insures that every pup is properly socialized in a home environment from the time they are born until they are placed with their new families.
Families who wish to participate in ________'s reciprocal arrangement must live within 90 minutes of ________, pass a screening interview and agree to the breeding arrangement. ________ provides the stud service, all veterinary care, some food supplies and any necessary medications. From time to time, conditions may allow some of these elite females to be available for $3000.
I'm not claiming that the kennel talked about above engages in that practice--I don't know them. It's possible that the kennel may be co-owners of some of the bitches they produce and that they breed their studs a lot. There are many different kinds of arrangements that can be made. But it is something to be aware of when you see a kennel which seems to have a large number of litters per year, many puppies to sell, and when the puppies seem to have been born in a lot of different places.