IIRC, because the payment terminals have their own power source, previous iPhone generations lacked the induction coils necessary to support passive NFC tags. Presumably they were added to the iPhone 7?
It could also be some kind of security thing where the 7 was designed to be able to have 3rd party access where the 6/6s would have required the secure enclave to do something risky.
Right but I mean that perhaps in the iPhone 7 they somehow rejiggered the SE interface in anticipation of doing this. That way the 7 hardware has a very secure way to do this where as since it wasn't planned in the older phones it might not be secure if they tried to provide access from user software. Maybe the NFC reader is attached to a different chip in the 7 that makes this easier/secure than in the older phones.