Because all your previous information is retained. A full restore will wipe every piece of data, including you Apple Pay DANs.
It will require you to set up Apple Pay again, but it won't matter because even the same card will acquire a different DAN (device account number) and hence won't be linked with the preorder.
Now I'm even more confused, my question was why wouldn't doing a full restore require you to set up Apple Pay over again. I guess I'm just agreeing with you guys.