Sure. How about, much like the reason many keygens don't work for PC games' online play, would be that Nintendo knows which Wii points have actually been assigned to a card. Don't know whether or not cards get activated at registers that sell them (retail-wise).
However, if you use a points code that hasn't been issued to the wild, seems to be a good way to get yourself banned from online *anything* forever with that console.
Just a thought :-p