Because the GC controller is the best in the world...
This is a valid point. The GCN controller has poor button layouts, but for a game like MKWii, that layout is actually good. Combine that with the fact that the GCN controller is the most comfortable traditional controller ever, and you basically come out with the fact that any game that CAN be played with the GCN controller SHOULD have that option.
Moving on, since we know that option does indeed exist, those graphics pointed out in the initial post are because the graphics will change depending on which controller method you have selected. If you've got waggle controls selected, the graphics show Wii buttons. If you've got the Classic Controller selected, it shows CC buttons. If you've got the GCN controller selected, it shows GCN buttons.