Forfeit rule when using Libero

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Four of us worked a local HS scrimamge last night. Frosh v Frosh, JV v JV, V v V, then a V v JV grudge match. One thing that all four of us had failed to realize until pre-game conference was that a Libero rule states that if a player is disqualified and the number of players drops below 6 the game is a forfeit. Can anyone tell me the reason and why this rule makes sense? I didn't attend a clinic this year, but the two who did said the issue was never discussed.