Note the verbiage on the second section of the supported issuers: "Add a card from any of these participating banks and continue to get the same rewards, benefits, and security of your cards."

I have a feeling that unless the issuing bank is supported, that Google is basically routing the payment through themselves. (Your responses on the other thread seems to confirm this.) So basically, Google bills your card directly, and they somehow route the money to the merchant. (Not sure what would the the "normal" way to do that; a bit out of my depth. I could probably ask people if you really want to know.) This means that, for instance, if your card gives a bonus for restaurants, and you use Android Pay and the issuer is not supported, that you likely will not get that bonus. Because according to your card the merchant is actually Google and not the restaurant.

So basically, Google is acting as a middle man in the transaction. They (somehow) pay the merchant in lieu of your card, then bill your card directly. This would not happen with a supported issuer; the payment would proceed as it normally would without Google and you get all your sweet, sweet reward points.