How is this relevant? The American Express Membership Rewards integration dates from at least 2011. Amazon needs to verify that you have a sufficient points balance to redeem, and have been able to do so since the outset. The notion that American Express is desperate because they lost one contract isn't convincing. They have long been good at monetizing information, and this is just an example where we can see it. Be assured that the other rewards cards issuers do the same.
For what's worth, my Amazon account presents the opportunity to opt IN to this feature:
Nope it's new the itegration is old but the sharing of POINTS information is new. It is relevant for two reasons:
1 the lost of costco in 2016 causes 1 in 10 Amex cards to be gone POSSIBLY a big hit on top of the 9% hit they took at the time. So this action smacks of desperation and AUTOENROLL is just another example of a worsening customer relations at Amex. We're out of there in 2016
2 Asking Amex if there are enough points to redeem ( ACK or NAK ) is fine but to actually give over the totals is another thing entirely. It can be used to profile in very interesting ways and Amazon ( I like them alot) will take full advantage of it.
A question for you, did the other issurers you mention ASK or autoenroll?