Yes, AA occasionally endorses tickets over to other airlines as a last resort. Obviously, AA would rather keep you flying AA, but sometimes it is not possible. For example, AA has a bunch of Hawaii service each day, but only one flight ex-SFO. In this scenario, it might be more advantageous for AA to put you on the other airline instead of incurring the expense of rerouting you (and potentially losing a valuable, sellable seat on the LAX-LIH segment).

Now, flying out of ORD on a domestic flight? That's another story. AA has many, many flights out of ORD every day (multiples to most cities), and it's a VERY competitive market for AA with UA. I'm not surprised that they refused to move you to UA.


While we're on this topic....
I also think that there is a difference in how domestic vs. international stations handle these rebooking situations. Once, I missed my connection ex-MAD because of a mechanical delay. I knew there was only one AA flight that day, so I suggested that they try moving me to another airline. The AA ground agents at MAD told me they do not move pax to other airlines, and said "come back tomorrow and we'll try to confirm you on tomorrow's flight." Hmm... that was not a reasonable option for me, so I thanked them, walked away from the ticket counter, and called EXP. The EXP Desk immediately rebooked me onto one of the multiple, wide-open IB flights nonstop to my final destination. ^ ^

Not sure what happened! Especially because IB is in oneworld, and my eventual ticket had an AA code! (why in the world couldn't the ticket counter have done that?)

But gotta love the EXP Desk! (It was worth the cell roaming charges!)