As explained so many times on the forum, and on the manual too, the airline codes of YOUR plane don't have any effect on the choice of the ground operator, but ONLY on the choice of the containers.
This because it would be wrong that your airline could have a chance to be serviced everywhere you go. So, the operator is correctly linked ONLY to the atc_parking_codes of the *parking*
In fact, American Airlines hasn't been assigned to KMEM with the whole 4 letters ICAO matching, so it doesn't have enough score to compete against Delta and Southwest, regardless of the atc_parking_codes of the parking, because they become a factor only when at the same score.
So, everything is correct and working to specifications, the only thing that might be discussed, is if it would be correct or not to add AA with the whole 4 letters score at KMEM, since KMEM is not an AA Hub (at least according to Wikipedia)