It's a fact that has been pretty well documented over the years, although I can't be bothered to dig out a source for this right now. Basically, frequent flier programs earn good money by selling miles to partners (e.g. Amex, Hilton) to give back to their customers and then these miles are used by customers to fill seats that would (hopefully) otherwise have been empty. I understand that in several US airlines, while the overall airline was making losses the frequent flier 'unit' was still making healthy profits.