I'm probably going to be in the minority here, but the airline offered a way to avoid having to be subjected to a middle seat (by having the passenger pay a $19 fee). I don't know if OP consciously avoided the fee by selecting a middle seat when the ticket was purchased or if the middle is what was assigned at check-in but in any event the $19 was not paid.

We can question whether or not the fee is just all we want, however if I had paid the $19 and then saw someone else move into a $19 seat for free I'd be upset and would be writing to AA to get my money back.

Now if people are asked to move because of a weight/balance issue then that's a completely different story.

Surely the answer isn't to go back to the pre-dereguation days where it cost significantly more to fly than today (even with fees added in).