From a court decision which is kind of an 'interesting article':
US v. AMEX, 15-1672, 2016 WL 5349734, NYLJ 1202768610015, at *1 (2d Cir., September 26, 2016)
In contrast to Visa and MasterCard, Amex is a proprietary network that operates a "closed-loop" system. Within this closed loop, Amex acts not only as the middleman network but also as both the issuer and acquirer for the vast majority of transactions involving its cards.26 Therefore, in most cases,27 Amex maintains direct relationships with both its cardholders and merchants by "provid[ing] issuing services to cardholders, acquiring and processing services to merchants, and network services to both sides of the platform in order to facilitate the use and acceptance of its payment cards."28 Special App. 16. The Amex system is sometimes referred to as a "three-party" system because the network (i.e., Amex itself) serves as the only intermediary between the cardholder and merchant.
Within its closed-loop system, Amex directly sets the interchange fee so as to maximize profit.29 Setting the interchange fee also allows Amex to set the accompanying merchant-discount fee and cardholder benefits directly.30 Amex charges a single discount rate for all Amex card products, meaning that regardless of whether a cardholder uses Amex's highest rewards card or its lowest, the merchant still must pay the same fee in order to accept the Amex card.
Unlike Visa and MasterCard, which run "lend-centric" models deriving more than half their revenues from interest charged to cardholders for unpaid balances on the cardholder's charges for a given billing period, Amex runs a "spend-centric" model whose revenues are primarily dependent on merchant-discount fees. This model is critical to Amex's merchant value proposition, which is that merchants who accept Amex gain access to "marquee" cardholders who tend to spend more on both an annual and per-transaction basis than customers using alternative payment methods.31 Amex's model is also critical to its cardholder value proposition, which features a robust rewards program and numerous other benefits, including customer service, fraud protection, and purchase and return protection.
______________________
I'm thinking that Amex's lawyers did a good job convincing the court but that it is no longer true