Research has shown that when a product is pretty much the same for everyone, and its repurchase frequency is high, one the most effective ways to spend marketing money is on loyalty programs. If airlines, etc., didn't have these programs, or if the programs didn't work, they'd need to do something else - which would cost them more.

So, think of it as doing them a favor!

(Slightly tongue in cheek, but the basic facts are correct.)