QF simply doesn't want the OWE ticket revenue for JFK-LAX (it isn't enough). So they have this rule that you cannot fly QF107/108 on the US leg unless you fly QF trans-pacific (which earns a lot more revenue). Cabotage has nothing to do with it; being able to fly QF codeshare on AA is a good example, that too would be cabotage if QF107/108 were -- but of course it is not! As it is a commercial decision, it is unlikely to change unless the market cools down and QF has empty seats to fill.