It's become standard practice in the travel industry to use frequent flyer miles as a reward when training consumers to change their behavior. So, for example, when the airlines wanted travelers to shift from booking via the carriers' call centers to making reservations online, they offered bonus miles to do so.
Northwest customers have a new payment option, PayPal, and to promote it, members of the WorldPerks program are being offered a 500-mile bonus for every Northwest ticket purchased via PayPal through September 30.
To qualify for the bonus, bookings must be made on Northwest's website, and travel must be completed by December 31.
Since PayPal's fee is absorbed by the seller—Northwest in this case—there's no extra cost to the buyer. So the extra miles are free and constitute a real bonus.