Why does it matter whether or not somebody uses nitrous (or anything else) in the Sunday Cup? Go ahead and break out the Mazda 787B if that's what floats your boat. Or try to win it with a maxxed out Fiat 500. No limits on entries in the Sunday Cup other than N or S tires.
Interestingly, the votes don't match the comments.
I think your in the wrong room or took a blow to the head. This has nothing to do with the Sunday Cup in GT4 the game.