I would assume that their profit equation works something like this: they need enough capacity to ensure that during the peak season, nobody is unhappy. If 90% of their contracts are in January, then that peak is huge, so they can't really skimp, even if most of those people never come back. If, e.g., 90% of signups were on the 1st of the month, it would be a lot easier: the peak occupancy would be much lower.