I don't understand this partnership (from a CS perspective) if Amex cares about the customer service that its Plat cardholders would be receiving from Expedia.
It's not a partnership. AMEX has outsourced most of its travel agency functions to Expedia as a cost savings measure. They certainly know of the problems it's caused to cardholders (since the problem cases ultimately wind up being resolved at the executive level in Phoenix). Presumably the cost savings are such that AMEX is willing to accept that level of cardholder unhappiness.