There's a huge difference in the amount of space you get in domestic 3-class F vs. 3-class J. And the difference is even bigger if you're on the 777 in F. Surely you noticed that those seats reclined to a fully flat bed? And that every seat had direct aisle access with no neighbor right next to you? That's worth something for many folks. That said, many find the 762 J seat to be pretty comfortable for recliners, but they can be tight, especially when the person in front reclines.

The catering is moderately better in F, that's been covered ad nauseum on these boards, but basically a custom salad with toppings, one extra main course option, better plating and custom sundaes for dessert. Definitely not worth $1000 (though the difference between full-freight J and F is quite a bit smaller).

And your ticket is a somewhat known loophole of LAX-SJU (or other Caribbean destinations) fares that AA has never decided to close.