Well because they are one of the main card issuers in the US and have cards useful in terms of benefits and the flexibility of MR points.

It's still not clear exactly where/how the error originated, and may never be publicly, but it seems that it most likely came from the way AMEX payment systems interacted with the AA payment system in currency conversion. AMEX acknowledges their systems contributed to the error and made it worse, but stopped short of saying it originated from them. They also told me it took extra time because they had to claw back ~$30M from AA to rectify my account because supposedly they already transferred the $30M to them. AA then pointed the finger at AMEX for the conversion error, but admitted the multiple layers of errors in their reps failing to properly investigate the claim of an erroneous refund. I suspect neither company will ever admit they they "caused" the error, but both have recognized they bear some responsibility for the misery of dealing with it.