I just arrived on 728 from LAX to DFW. My first time flying on AA since the started charging for seats.

I was in a middle seat. Boarded and the aisle seat next to me was empty. There was someone in the window seat. Once the door was closed, I decided to move over to the aisle seat. The FA came by and asked to see my boarding pass. I showed her and she said you belong in Seat B, not C. I said that in the past, when there was room, I would also move over and it has never been an issue. Then she asked if I paid the $19 fee for seat C. I said no and she said that I have to move back to seat B or I will be removed from the plane. She said it would be the same if I tried to move into first class.

Seemed pointless to me. Besides, it is not like moving into first class. In FC, you get more (hence you pay more).