Except (pardon my directness) it isn't an open jaw. "Open jaw" has a specific meaning (and there are many many pages in the ticketing manual describing what it means).

For a start, an open jaw requires two fare components [excluding end-on-end and side trips]. But RTW itineraries are considered as one fare component.

You also have the problem that a RTW fare is published as CAI-CAI (for example). If you want to allow trips that start at CAI and end in (say) AUH, unless you have a final surface sector, you now need to publish fares on CAI-AUH, and on every other combination.

Adding a final fare break surface sector solves that problem. Open jaw pricing does not.