The reason breeders put this in their contracts is to protect their lines from being indescrimentatly bred. Companion yorkies are sold for less than those that are sold with unlimited registration. Those sold with unlimited registration are sold to people that the breeder has full trust in knowing that their lines will be taken care of. It's really up to the breeder as to how they'll allow their dogs to be sold.
This type of contract can be enforced. But, as stated by someone else, most responsible, reputable breeders are now spay/nuetering their companions before ever leaving their homes.
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