On a related note, and just sharing my experience as best I could tell how the gate agents handled the flight we took back from CDG - JFK a couple of years ago:

The Delta flight had enough BC seats available that I thought my family and I would all get BC. It appeared the gate agents put coach fare revenue passengers up in BC, and the non-revs got coach. I'm not sure if this is the experience others have had, or if it was a one off. I would have engaged the gate agents, but at that point I was happy to just be getting all of us on the same flight home and the French laddies had an air of disgust for all things American about them.