Don't most payments that are done using the iPhone, at least those made to Apple services, require the user to double-click the side/power button to initiate the process, which would be the button used for Touch ID with iPhones that don't have Face ID? That's been my experience. I don't use my iPhone 17PM to buy much of anything other than Apple services through the Wallet app, since I prefer to make other purchases using my Macbook, so I don't have a lot of experience buying other things using an iPhone, though a couple times I've used my iPhone to make a contactless payment at Target and Whole Foods, and even there I had to press the side button twice to start the process, and Face ID came into play only inbetween those button presses and tapping the iPhone onto the payment terminal.
So having to tap the side button twice to start a payment, on an iPhone that has Face ID, gives you at least some non-Face ID control over the process, sort of like having to apply your finger to the button to read your fingerprint on the iPhone Air.