Their response to you while you were leaving has everything to do with them knowing exactly the purpose of you buying that rat and the person who said that not being mature enough to handle it a different way.
Unless those specific employees are stupid enough to do something crazy on their own, PetSmart itself doesn't have any sort of policy to do anything to their pets that would cause your snake harm by feeding it to them. It's simply against their policy to sell their pet rats as feeders. Always has been...