I think the idea is that when a bird is wounded and it flies away, it's likely to be eaten by a larger raptor. Any lead pellets left inside are then ingested by the larger species, of which most are federally protected.My argument is that in one weekend, a single Gold Miner using a Suction Dredge can abstract between 600 and 1200 pounds of sinkers from a stream! My shot gun blasts are minuscule compared to that.
I'm not claiming to know if it's true, or exactly how lead is moved around the environment, but I don't think the issue is lead left lying on the ground or in the water.