I usually still see roundtrip more expensive than one-way, but generally the difference isn't anything big. US<->HK would cost say $800 while one way would cost maybe $700. My Speculation is that they want to make sure all the flights are relatively full. If you have a lot of people going from HK to US but none the other way around, their profit would drop since they have to get the plane back anyways.
But yeah, I find that very annoying when I just want a 1-way ticket. Could you get the round trip ticket and screw the return ticket?