I've never used one of these so haven't actually paid attention, though presumably there's no meaningful way to track this. The merchant has no idea you have a credit coming from AMEX and AMEX isn't analysing or reporting tax treatment of individual transactions.
Perhaps there's some sort of caveat in tax law that prohibits a deduction where a benefit like this is attained, though if there is, the onus isn't on AMEX or the charitable organisation to tell you that; it's on you to know it. Perhaps "helpful" if AMEX chooses to tell you, but they wouldn't have to, so the omission wouldn't technically mean anything.