It sounds plausible. The vouchers are usually endorsed to say that they are valid on AA only, so AA accepting them for non-AA segments is AA doing you a favour. In return they can expect some benefit, and on the RTW fares the carriers flying trans-Atlantic and trans-Pacific get a disproportionate share of the revenue (a quirk of the OWE rules). In this context I would fully expect AA to require this, and you have no recourse if you want to use the vouchers. That said, flying AA over the water can be quite nice, in fact I have willingly flown AA on an AONEx ticket in preference over BA -- so despite what you hear on the forums, flying AA isn't that bad, and on some routes it can even be the optimal choice.