I think, that the information provided here is true. From the information I have gathered from sources that are running their small retail businesses, my take is that VISA is offering charge in tiers - for example(number are exagerated and hypothetical): tier 1: 1- 10 000 - 3,5% tier 2: 10 000 - 100 000 - 3% tier 3: 100 000 - 1 000 000 - 2,5%

etc. you get the idea, so if VISA credit has low transaction count, then expenses for the transactions are a lot higher than for VISA debit transactions - if the user base for VISA credit is couple of tiers lower than for rest of cards, it makes sense for these "rare" users to switch to other cards, so Amazaon does not have to subsidize their loss of income from VISA credit cards.