The problem with saying it's BA or IB's problem is that in situations where I've had mixed tickets (even on one PNR) involving only a few segments on AA, once I've only got AA operated segments remaining on my ticket, BA and IB (at least, my experience in these situations is only with them), usually take the attitude that it's AA's problem, since all remaining segments are AA operated at that point (I don't say that this is the correct interpretation, only the position they take).
I have had AA honor the the guarantee coming from a non-AA (but oneworld) ticket to an AA ticket. In both (or maybe all three, can't remember the details of the ticket on the third one) cases, however, I had to go outside of security to the check-in desk and make something of a nuisance of myself until I eventually talked to someone who knew how to make it happen.
I think this is a great policy, since the reality of the JV agreements mean that there is often not a practical way to book a single ticket using only one carrier metal (especially between e.g. Southeast Asia or Eastern Europe) and non-major cities in the US.