If you try to book a ticket ex-Philippines on Asiana's website, you get this message:
According to Philippines government law, ticketing for flights departing from Philippines should be purchased there.
In conjunction with this, ticketing for flights originating in the Philippines are not available on the Asiana Airlines homepage.
If you wish to purchase tickets for flights departing from Philippines, please contact Asiana Airlines sales office.
A similar message is displayed when trying to book via Korean Air. Not sure what other airlines also restrict such purchases, but I know some do not. Perhaps there is a law that is open to interpretation and some airlines interpret it one way, and others another. The Philippines is infamous for shooting themselves in the foot and doing their utmost best to discourage all types of business, so it certainly wouldn't surprise me one bit if there is indeed a law restricting people not physically in the Philippines from purchasing a ticket there. I don't know of any travel agents in the Philippines that will even accept a credit card for purchasing tickets. They all require cash only, I guess due to huge amounts of fraud. Some airlines will allow credit card purchases, but there's often headaches involved and frequently foreign issued cards are prohibited.
My best guess is that whatever this law is that Asiana is referring to has likely affected BMI and they decided to simply completely ignore the Philippines rather than run any risk of running afoul of some insane law.