rnvinc,
Yes...it's to prevent heavier jigs such as bass jigs from pushing the lid over and hooking the rope. A lighter jig, being trolled probably wouldn't move the lid enough to hook, but some of the heavier jigs and carolina rigs may possible do so. That's also why the rope runs down through a hole in the brick and is knotted on the bottom, to prevent a dragging lure from hooking the rope should it be tied on the outside of the brick.