In this instance though, the original buyer is no longer in control as to whether or not the cat can be spayed. So, what is the penalty for the breach? Specific performance is not possible, and how is the seller financially harmed?
A penalty for the breach can be the reclaiming of the animal. Other remedies can be specific performance or an award of damages (for, say, devaluing the breed).
Some breeders will hold an animal's registration papers until proof is provided by the buyer that the animal has been spayed or neutered. If the buyer does not return in 6 months (or whenever) with this proof, the buyer can be sued for breach of contract and the animal repossessed.
Many breeders will only sell AFTER they have spayed or neutered the animal, to eliminate the problem.
But with any contract, the contract must be read in its entirety. There could be remedies spelled out, for example, that outline what happens should the terms be breached.
The bottom line is that these clauses in breeder contracts are enforceable.