I doubt sellers will want to accept it. You can't pay your taxes with Bezos-bucks.
I still don't see a big upside for Amazon, but merchants wouldn't need to accept it. And of course, Amazon could incentivize merchants to use it. Maybe if you take it, you get 1% more value than if you take USD and spend it on the site/AWS.
Imagine if the dollar fluctuated widely (say because of inflation) but BezosBucks does not. Your buying power stays the same (while BezosBucks becomes more expensive to buy into over time).
If they send me 101 Bezobucks instead, I'm happy because they gave me $101 of value; they're happy because for that $101 purchase, they got $100 value; if I paid with a rewards credit card, they would probably get $98-$99.
Why it would need to be a cryptocoin thing instead of just a gift card/credit, I don't know. Cryptocoins are hot for reasons that don't resonate with me. Maybe some way around gift card laws, and if they can back into foreign remittances without needing a money transmital license, it could be useful.
I doubt sellers would say no to a sale if a client offered to purchase it with a blockchain store card.