
As I understand it, please legal minds tell me if I am wrong.
I don't think it is that contracts aren't legally binding (other than what you can do with you own legal property) but that to enforce them you would have to take court action, costly and no surety of winning.
Mainly the contract will say what the purchase entailed, and what it does not. this should be that the puppy was sold as a companion, healthy at time of sale, and as healthy as was possible for the breeder to predict based on health tests available for the parents for any known hereditary defects.
For example if you were foolish enough to sell a puppy as a show winner and breeding animal (as opposed to a companion with potential to be either of those), then if it did not develop and do as well as hoped for in the ring, or for some reason was unable to reproduce, or was unsuitable to do so then the buyers would quite rightly say they had a case against the breeder for selling them something 'not fit for purpose'. Same thing if the puppy went onto develop a health issue, if that had been covered in the contract that all health tests had been done with good results and that is as far as any health guarantees go, then one would hope no reasonable judge would find against the breeder (though it has happened where an owner pleaded ignorance of the possibility of HD).
the only way a breeder could retain any legal hold over the actual ownership side is if the dog was loaned or leased to the new owner, or owned in partnership, and even then civil courts tend to look on who the dog has lived with been looked after by etc.
Many rescue centres retain the ownership of the animal yet are still powerless in any practical sense if the adopter gets rid of the animal to a third party moves etc.
So the return to breeder clauses are really only a statement of intent and responsibility by the breeder, a suggestion.