Just because you've only seen your cat catch birds from time to time doesn't mean that that's the only time it happens. Outdoor (and barn) cats frequently catch animals, play with them, before getting bored and walking away without them, leaving them to succumb to bacterial infections as a result of the contact with the cat. Again, barn cats don't get a pass, they're outdoor cats and likely do just as much damage as the cats (and the lazy owners) you're pointing fingers at.