I've just found another link that isn't paywalled: https://www.reuters.com/business/retail-consumer/amazon-stop...

This says that only Visa credit cards will be blocked, not Visa debit cards.

I have a speculative guess that it might not just be about the merchant cost... It does seem that Visa offers more customer protection than Mastercard from what I can tell, so perhaps too many customers have got Visa involved for refunds because of the many dubious Amazon marketplace sellers listing fake or substandard products.