I think that with other airlines dropping this CO no longer sees offering it as a competitive necessity. Also, CO will offer bonuses, etc. if it thinks that by doing so it can get people to act a certain way. The number of CO.com bookings has clearly increased (for a variety of reasons, this promo probably being one of them). I guess they don't feel they need to entice people with this offer because it doesn't change behaviour as much as it once did.