LAX-NRT is the most competitive transpacific route served by the largest number of airlines, I think. SQ's around USD1,800 for a round trip out of Tokyo is still available. Its just over 1,000USD if you wish to fly MH on this route, a round trip, of course. SQ is cheapter than NW on this route, too! Yes, it's a fifth-freedom route for SQ. That's probably the reason.

You can buy the cheaptest distance/price wise fare with SQ (as well as all other airlines) in a certain Asian city .... I am not talking about BKK, where, I think, SQ does not sell so cheap (except economy tickets). SQ is also not bad out of TPE, too, pricewise.