This type of benefit costs American Express nothing. Saks may even pay to be featured. American Express isn't going to turn away these types of opportunities, which will keep their card in some wallets, just because the Flyertalk demographic doesn't value it.
Amex said that Saks is paying the entirety of the $100 annual credit.
The CEO said that Saks approached Amex offering to foot the bill to get access to Platinum card holders.