Don't know what they do over there, but we usually get the lungs and guts out as soon as possible in order to keep the meat from spoiling. Long lived predators that likes to scavenge can develop lead poisoning from those remains if it's their main source of food.

If confusing with rocks was the main source you'd expect it to be just as common in other birds.

This is untrue, gastroliths are associated only with birds that eat plants. They grind up food, which isn’t necessary for meat. Eagles eat bullets from animals that have either been shot and abandoned, lost, or had parts of them discarded as zqxwas pointed out.