It is not new, that AA wants the agents to reaccommodate pax on their own metal - or at least joint venture partners.
Back in my days, we filled up BA to LHR very quickly and then rebooked on everything, that offered a reasonable connection - from the top of my head, I can remember booking pax on BA/AB/DL/LH/LX/OS/LO/SK/AF/KL/OK/IB.
Oh, WFRIS1-2ĄC1 - those were the days!