That's a common way of doing it, but Apple devices actively amplify the signal in card emulation mode as well, which gives them longer range than physical cards or "purely passive" devices.
But it also means they can't do the neat trick of paying with a completely dead (i.e. not even reserve battery power) phone that some early Android and Windows Phone devices could do.
Maybe I’m not understanding properly, but iPhones absolutely can do NFC payments when the phone is dead. Your nominated “express” card will work for transit payments, and I believe car and house keys continue to work too.
No, that still requires some battery on iOS, i.e. it's only possible in the same "power reserve" mode that still sends the occasional "Find my iPhone" Bluetooth beacon.
Field-powered mode is possible in at least some NFC chipsets, but I suspect that Apple either values a consistent NFC range more than usability even with a completely dead battery (the amplifier that grants a significantly higher NFC range to Apple Pay obviously needs power), they see it as a security feature (reserve mode is capped to a few hours, I believe), or their NFC controller simply doesn't support it.
If you think about it, it's still more secure than a physical card – that also doesn't have any authentication method at transit terminals, but unlike the "Express Transit" option on iOS, you can't turn that functionality off at all.
But it also means they can't do the neat trick of paying with a completely dead (i.e. not even reserve battery power) phone that some early Android and Windows Phone devices could do.