I just ran into this the other day in STL. The gate agent asked me if Delta was "unlimited" on American. After a yes, she said, "well, I better check"....whatever...so after she did what she had to do, she explained what the unlimited meant. I think limiting the jumpseat is because it costs the airline some small amount of taxes/fees/other BS surcharges paid to the airport. Similar to why AirTran was charging for the second off line J/S.