There's the danger of mercury poisoning and if you were to only feed canned tuna to your cat, they wouldn't get all of the nutrients they need. This is very unhappy news for Juniper the Cat, who only has cans of tuna on her Christmas wish list.
Not everyone is allowed to live in military housing on Fort Bliss in El Paso, Texas and the same goes for dogs. If they applied these rules to people though, they'd be sued in a millisecond.