If they're an AA customer inbound, I don't see why they'd be excluded from using the bus for a different airline's flight.
I've taken CO mainline to EWR (Terminal C) and then connected to B6 mainline (Terminal A) and the contract worker did not allow me to use the air side shuttle since it was for onward Continental Express passengers only. Pretty ridiculous since I flew CO in and at the time was OnePass Gold but that's EWR and NJ for you. Where do these buses arrive and depart at the mentioned gates? There is no waiting area since AA doesn't have any operations at remote stands or terminals at ORD.
Josh