PetSmart is a privately owned company. They're under no obligation to make sure your snake is healthy. Just like they're under no obligation to make sure your dog is healthy. After they sell you a critter, that's pretty much the end of the transaction. They don't have to provide you with the ability to feed it just like they don't have to provide you with vet care.
Really, if you think about it, that's a GOOD thing. It's called Freedom of Commerce.