Aaahhhh...but FF programs were never intended to be logical in that sense. I'm serious.

The programs were founded in the early 1980's. The target audience at that time was business travelers - much moreso than today. Airlines know that most business travelers are allowed to pick which airline they fly within reason. They also know that most business travelers cannot ethically accept a straight cash kickback. Hence the loyalty programs with a fake currency that is very detached from cash.

It became culturally and ethically acceptable for business travelers to keep their miles, whereas had they been hard-linked to the fare paid, that wouldn't have happened. The result was that biz travelers had every incentive to stick with your airline, because the rewards were essentially "free" - and quite lucrative.

For leisure travelers, as we all know, no miles are free: their cost is baked in to anything you buy that generates miles whether it be flights, flowers, or a regular credit card purchase. But it's the business travelers that got this whole thing rolling, and for that to happen the programs had to stay as far detached from something that looked like a "kickback" as possible.

The people who thought this up 25 years ago were pure marketing geniuses. (I mean that in a good way - it's been a very nice ride for people like us who became fans of the programs whether through biz or leisure travel.)