You will see this on almost every airline's website. If on an airline booking flow, country is a drop down for nationality, billing address and phone number. All of these require the split due to different passports, different postal systems and different phone country codes.

As for the drop down on the flight search page, this is to direct you to the correct point of sale, since Mainland China, Hong Kong and Taiwan all have different currencies and different tax regulations (if a local BA office is registered).

It is practical rather than political.