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.
Before you call this guy a "hero" for what he did, consider for a moment that whatever this guy's likely motive was you can be sure it had nothing to do with wanting to "help" the animals.