At most airlines, miles purchases, gifts or transfers are billed by points.com, not by the carrier. The charge must be submitted by the airline to be considered.
DL's miles purchases are posted as DL, appearing like a ticket, not by a third party vendor. Not sure how AMEX's systems are set up in terms of detecting if the transaction is refundable. IIRC, the "ticket" type is in the miscellaneous category, similar to bag fees.