My thoughts regarding those who say that people who pay more per FF mile earned are more somehow more deserving than those who pay less.
The airlines give us the rules, and I decide which routes/airlines to fly based on the FF program and miles earned. Some of us may be more of a master at earning miles than others. For instance, I can become an elite member on another airline by sending a letter requesting elite status and by showing I am already an elite on another airline; perhaps this isn't fare to those that had to earn their miles on the airline I am seeking elite status, but all things are not equal. If airlines want to change the way we earn FF miles, or the amount of FF miles we earn, then they are free to do so; they make the rules, and I play their game. It is the same when shopping, buying a car or house, etc...
My thoughts on why airlines don't directly give FF miles based on the amount paid for a ticket:
Yes, on the surface it doesn't make since. If I pay $500 for a ticket and fly 1000 miles, shouldn't I get a bigger FF mile credit than if I paid $500 and flew 3000 miles? After all, it costs more to fly a person 3000 miles than 1000 miles. But I did some thinking and came up with the following:
1. It would give the IRS an easier way of calculating the FF miles value. Business travelers and Congressmen would now have pay taxes on miles earned when someone else pays for the ticket.
2. It would make it easier for companies to negotiate with airlines a discount equal to the value of FF miles in exchange for forgoing the FF mile credit. Again, business travelers won't like this.
3. When a person is reimbursed for travel expenses, it could actually prompt some travelers to intentionally buy the more expensive tickets so that they would get a larger FF mile credit. This won't go over very well with businesses and local governments.