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).
You paid $19 to guarantee and select a premium seat. If the plane had been full, many people probably would have been placed in 'premium' seats for free. Would you be entitled a refund then? IMO the FA went way over the top and I have never seen this before.
I moved to an exit row after takeoff once (pre status days). FA stopped by and just said she needed to confirm I would perform functions if required since I wasn't there during the initial briefing.