Usually this is because they are expecting an equipment change, but they can't put the new map up until it's official. I have had this happen numerous times when booking flights and sometimes well afterward. The phone folks are usually of little or no assistance. You can go ahead and book the ticket and then check daily for your seat map in the days and weeks following... I once had booked a TA trip with connections for a client and it wasn't until a few days before trip that I could pick their seats for the last TA leg. But because of this tendency by AA during a time when so many planes are being retrofitted and swapped on and off routes, I make it a point to monitor my seat even after I have already selected and/or paid for it. There have been other occasions where an equipment swap has been made and they will never notify you of that. If you are picky like I am, you will be none too pleased if your carefully chosen MCE seats or your nice single (rare) pair of coach seats suddenly turns into something else entirely.