I remember HAL doing that, as well. Their rationale was that they wouldn't know if they had enough for a charter, so they'd leave open to individual bookings until that point, just in case. I agree that it's unethical and sleazy. It should be that the chartering organization needs to fill the stated number of cabins by a stipulated date, and if that's not met, they lose the charter and it gets opened up to all.
Question for cantwait2go -- did NCL offer a credit that would cover air rebooking fees if you opt for an NCL cruise on a different date?