Your analogy is inappropriate. Would not knocking over piece violate the rules. If however you and I play and I agree to let you do that I cannot cry later that you cheated. I agree to it.
Look at a better analogy. I play a game of 9 ball and I agree to let you have the break and the 7 ball. If you break and make the game ball, you did not cheat.
Or at 8 ball, I offer to let you choose any three balls after the break to remove. You select them, you did not cheat, I gave you a spot a handicap. If however you just take three balls of on your own volition, that would be cheating.
I do that all the time. My most enjoyable wins are giving the queen away early and winning.