That was pretty much Jetblue's policy. But, look at it from their viewpoint. People booked the flights they wanted, and then hoped and prayed the fare went down. As soon as it went down, they got on the phone and called JB...who in turn, allowed them to get the entire difference in a JB credit, to be used on future flights. I was able to make several flights with only a small oop cost simply because I had so many credits in my account.
I'm sure it got to the point that JB was losing money on the whole process...people booking at $139 and then wanting refunds each time the fare dropped, which it did in order to entice others to book a flight.
I have no problem with any airline asking for a change fee. Just last week, I decided to stay at WDW a week longer in Dec. I had booked my return flight, on a Friday, at $94. Well, I figured I would be eating that $94 when booking a flight for the following Friday. But, it was 'only' an addtl $100. But, when I changed my itinerary, online, it cost me only $50!! The fare was exactly the same, so no difference there. Just paid the change fee of $50 and got exactly what I wanted.
Fares going down is the chance we take when we book early rather than later. The airlines don't charge us if the fares go up....I always wonder why they feel they need to credit us when the fares go down. I guess it's good business for them to do that....up to a point anyway.