There is nothing in the fare rules regarding payment method (such as requiring any credit card used to be issued in the country of origin)
What is in the fare rules is the following:
When travel originates in a country for which a specific local currency fares is published and the ticket is sold in another country, the fare will be that published for the country of origin converted to the currency of the country of sale at the bank selling rate. The resultant fare must not be lower than from the country of sale.
Exception: Not applicable when BOTH travel originates and sales are made within Europe.
AA's agent in Cape Town (mindpearl) has for while required South Africa credit card for itineraries starting there. I believe yours is the first report of AA now requiring this for ex-Japan itineraries.
The online tool does not enquire where you are located, and seems to just assume you are in the country of origin.
As for online bookings that start with BA, from South Africa at least BA does not ask where you are located and accepts non-South African credit cards. There has been confirmation of this just today (see today's post by
Timbuktu in this thread -
http://www.flyertalk.com/forum/onewo...l#post26301562)
I would expect this to be the same from other points of origin, such as Japan.