You might say: but that won't change who actually wins on the private board. However when you use techniques like that you can get identify data in the test set and use it for training. It does help a some, but not a lot. The payout isn't very large for the over fitters (maybe 10-20% of payout).
Another negative is regular competitors also have no idea how they are actually doing compared to others. The leaderboard has impossible (for market data) logloss values of 0.2 - 0.4, sometimes even 0.00. They keep trying to fix it, but they haven't in about 2 months.