The plug is part of the federal migratory bird restrictions. My guess is that by participating in the activity you are required to show that the gun meets the criteria of holding no more than 3 shells. The most I have ever had to do was unload and show I could only load 2 shells in the tube.
SUMMARY OF FEDERAL REGULATIONS
In addition to state regulations, the following federal rules apply to the taking, possession, shipping, transporting and storing of migratory game birds. No persons shall take migratory game birds:
● With a trap, snare, net, rifle, pistol, swivel gun, shotgun larger than 10 gauge, punt gun, battery gun, machinegun, fish hook, poison, drug, explosive, or stupefying substance;
● With a shotgun of any description capable of holding more than three shells, unless it is plugged with a one-piece filler, incapable of removal without disassembling the gun, so its total capacity does not exceed three shells. This restriction does not apply during dates states have selected under the Conservation Order for light geese (i.e. greater and lesser snow and Ross’s geese) or those selected for the control of resident Canada geese.
Page 60 of the regulation digest.
http://www.ncwildlife.org/Regs/2011_12/documents/2011-12_Hunting.pdf