Originally Posted by
Often1
The answer is simple: money. AA does not provide Flagship access to its own Platinums (Saphire) on domestic itineraries (including in F) and other OW carriers pay AA for access for their Saphires. Thus, $0 cost to AA for its own Saphires, while BA pays for a BA Saphire to enter.
Out of interest, who decides whether Sapphires are entitled to access? Is it a
oneworld decision?