There's an important physical and physiological difference between a chicken swallowing a mouse whole, and a chicken swallowing a bone off a chicken carcass. The mouse, being whole, is nice and smooth on the outside, with the sharp little bones inside the body where they can't puncture anything on the way down the esophagus and through the crop. The bones only come into play once the mouse reaches the gizzard and gets ground up, but the gizzard has a very tough lining (meant to protect itself from being ground up), so a bone can't cause damage in there. A sharp piece of bone off the carcass goes down on its own though, pointy ends sticking out, through the soft esophagus and the soft, floppy bag that is the crop. Good opportunities for stabbing there. So, we can't say that eating exposed sharp bones from a chicken carcass is the same as chickens swallowing mice and lizards during their natural foraging behaviors. It's not a fair comparison.
