Well, the current pricing doesn't quite make it worth it in this case, but clearly if USA-Japan-BKK for example flights are $392 one way (which they are right now), you might as well get off the plane in Tokyo and throw away the 2nd leg. If only the return from Japan were as cheap, which generally it is not, lately.
The one-way BKK-Japan-USA is not $392 one-way. Fares don't usually work that way.
And anyway, how would you get back? You wouldn't be in BKK. To do hidden-city ticketing on the way back it would be Japan-USA-xxx
You would also get no checked baggage.
It would be cheaper just to buy a ticket to China, as someone else pointed out, and get a cheap ticket to Japan from there. It would waste a lot of time. And that's why there is a premium for flying direct.
But Japan has historically always been more expensive to get to than the rest of Asia. Budget carriers have started though, particularly KIX and NRT T3.