Oh well. Guess now I know.
I had always purchased tickets from SFO -> HKG. None of the tickets resulted in any foreign transaction fees. But this time I redeemed the miles ticket from SFO -> HKG as well. That's why I was not expecting to be charged in foreign transaction fee.
Anyway, I was just kinda disappointed that
1. Took me like 2 weeks to figure out how to contact an agent
2. For such a small amount, they can't compensate me anything.