Because they booked and paid for their ticket and seat in good faith and don't deserve to be removed to accommodate another customer just on status.
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The platinum also booked their ticket in good faith. They seem happy enough to apply this rule for a Platinum on a Full Y Push, so why not in this situation. The full Y Push displaces the non status flyer (who purchased their ticket in good faith).

If we were merely talking about a few hour delay for the plat that is one thing, but I had a few folks stuck for 3 days in Florida all of which were plats and something like 100 Continental flights departed out of their local airports (TPA, SRQ, MCO etc) before they could get re-accommodated.