Another reason 99% of reputable breeders will put a spay/neuter clause in their contract for pet quality pups is that it is their reputation on the line. Sure, the puppy now belongs to the new owner, but the breeding that went into that puppy is the breeder's life work. Something they would like to protect.
They have spent years and years researching the best matchings, spending more money than they could ever make selling puppies on health clearances, showing, breeding, etc. They have knowledge about the genetics, are experts on the temperaments of the dogs behind in the pedigrees, etc.
For example, let's say that the breeder has worked diligently over numerous generations to create a puppy with a very stable, family friendly temperament. Their livelihood is based on their reputation that their dogs not only have great conformation, free from genetic defects, but also have even, stable temperaments.
Now factor in the popularity of the breed, such as the pom listed here. (It happens in all "popular" breeds). The pom is one of the darlings of the puppy mill and backyard breeders - being cranked out to pet shops without any regard to temperament, breeding, health issues, etc. This accounts for so many reports of Poms being snappy, temperamental, fearful, etc. And all of these poorly bred samples of bad temperament, bad health etc, are sold with no restrictions on breeding.
So, the new owner, decides to breed their new dog to the cute pet store pom down the street. Being naive, the new owner has no idea why the two should breed - never putting all the research and money into the breeding that the breeder would. Maybe it was for the "children to witness the birth." Or they want a puppy just like their Precious. So they breed the two. The puppies will now carry the all the breeders hard work and ethics on their pedigree. But because the puppies are half puppy mill, they can now have bad tempers, bad conformation, etc. This, at minimum, could reflect badly on the type of puppies being bred by the original breeder, and at worst (if the litter bites someone) could destroy everything the breeder has worked for.
So, when you are buying a puppy, you are not only buying the actual puppy, but the knowledge, hard work and wisdom of the breeder that put that puppy on the ground. This goes for a reputable breeder right down to the backyard breeder or puppy mill. Just in the case of the BYB/puppy mill, you are buying the lack of detail to attention for the breeding.
If the breeder thinks that the puppy is not up to the standards of breeding, then they will put a spay/neuter claus in the contract. They don't want the owner to take a sub-standard specimen of the breed (although a wonderfully good, sweet, and better bred than most dogs) and have it become the foundation stock for a backyard breeder. No responsible breeder would let their puppies live that life. If the puppy were breeding quality, they would either have kept the puppy, or sold it on a different kind of contract.
If you intend to breed, there are certainly puppies out there to purchase. But they will cost you much more than a pet quality puppy. And you will have to agree to the standard testings (hip clearances, cerf, etc) You will also have to agree to show the puppy to make sure that other experts in the breed think the dog is a one of the best representatives of the breed.
And I want to add that I don't think the OP is intending to breed. I just used the Pom example because her breeder DID include a spay/neuter claus in the contract.