Originally Posted by denCSA

Let me try to explain what you encountered. Reservations agents (over the phone) see a very different version of SABRE than a CSA at the airport, and in turn the SABRE powered website you use when booking a ticket. The SABRE version in which RES agents use is highly restricted in functionality for many reasons. One of these restrictions is the seating application. RES agents can pull up a seatmap but it will only allow assignments for which there are hard data elements in the PNR to qualify that person for the seat (FF tier gets you a premium designated seat).

I know that online you can book whatever seat you want but you need to realize that SABRE is a technology based on decades old code, and even though the ala-carte system is in place nowadays, the computer system is still playing catchup. The website is likely accessing content through an API based on new-generation technology that can handle the new ala-carte approach, while SABRE is still a reservation system that is based on rules and logic for which F9's new system has not been adapted yet. Unfortunately, it will just take time to iron out all the details.