The CO/NW situation is clearly a capacity management situation.

Carriers that have local traffic rights between foreign city pairs often sell at below the price of the locals. This means that UA and NW have huge demand for cheap seats between, say, NRT and HKG or NRT and SIN.

But if you sell the whole plane on cheap intra-asia tickets, then you have no space to sell a North American originating passenger who actually wants to get to the farther point.

So they hold on to NRT SIN seats to marry up with US inbounds, which are not avaible to connections or originating pax at the intermediate point.

Similarly, you will often find flights to the intermediate point sold out, but available to the farther point. This is almost invariably the case on SQ2 between SFO and HKG. It will be sold out in P and J, but there will be plenty of seats to SIN.