If you always pay down your credit card debt every billing period, you're considered a "deadbeat" because you're using the benefits of the credit card's payment processing but not paying in interest.
The best customers of financial institutions aren't those whom are financially responsible or flush with cash, but those regularly incurring 14+% interest on every purchase and paying overdraft fees.
For airlines in particular, the deals they allow through credit cards are more like loss leaders. They need to fill up seats because that's the nature of their business model. If you don't fill up enough seats to make up for the operating cost, the plane doesn't fly, and if the plane doesn't fly then that's one fewer route you'll have paying customers for. Airlines need to keep routes going, so they create incentives to keep people flying, which hopefully gets them into the habit of flying again even when they've used up all their rewards points.