I understand all you are saying. However, The things Flounder eat in their natural environment have been eaten for eons. Their system has adapted to these food supplies. Although we all know clams have shells, I have never heard of a flounder breaking a shell to get at the meat.
As for the pissers, the digestive system of the Flounder has adapted to this and other natural food sources. e.g clam necks, small invertibrea such as grass shrimp, etc.
As for corn, rice and saw dust, they all have a different chemical and protien make up them what the flounder has avaliable to them naturally. Simply said, their systems are not capable of digesting, corn kernel husks, rice or saw dust. These are alien to the fish and pose a potential lethal diet.
I'm sure these chum ideas kill far less fish them the draggers, but they do kill.You just don't see it.