I thought that all parties and destinations in a joint venture had to enjoy Open Skies with each other. Does this mean that China is included in the UA-NH, DL-KE, and AA-JL TPAC joint ventures, all three? I have read many times that it does not apply to DL-KE.
UA/NH and DL/KE JV do not include China. And to another post above, AA does not codeshare with JL on NRT-PEK and neither does UA with NH on NRT-PEK. Same goes for DFW-PEK, JL does not codeshare on it. Of course, all carriers can sell a US-NRT-China ticket (and I have purchased them before), but they are unlikely schedule/revenue coordinated and thus not part of the JV.
It's good yield management to not occupy otherwise profitable seats with low yield connecting traffic. There is plenty of capacity to China
If you really want to do it, you can likely multi city it as 2 one ways, but again that would be expensive.
If China was part of the JV, this type of yield fare issue could be easily mitigated by modifying the fare rules to not allow TYO transfer/stopover or by only allowing only AA-operated segments, or even restricting inventory on TYO vs. China flights. As another poster noted, adding/removing codeshare is not a practical solution.