Personally I would diplomatically tell them to take a long walk off of a short pier and if they want to take the cat back they'll have to get a judge to rule that I was in violation of their contract. You, new cat owner layperson, have no knowledge of PetSmart's adoption policies other than what is in writing on that contract and they can't hold you to a stipulation that isn't on it. Further, if anything in the contract is ambiguous then that works in your favor since PetSmart drafted and presented the contract, see https://en.wikipedia.org/wiki/Contra_proferentem.
If their policy is not to adopt out cats to homes with snakes then they need to add that language. Heck, PetSmart sells snakes - what if you purchased one from them the day after you brought the cat home? Would they refuse to sell a snake to you because they have it in their records that you adopted a cat from them? And if snakes are so dangerous to cats why would they keep cats and snakes in the same building?