I'll be able to get more info later when my other half is back, but I'm 99% sure this is not the case. This is nothing to do with fees, it's Amazon bullying Visa with strong arm tactics, knowing that if negotiations fail they can do exactly what they've done and push people into getting the Amazon Mastercard. Amazon undoubtably has the lowest card fees of anyone, ever. The fact they are using high fees as an excuse is verging on libel!
Santander have moved their debit card offering to Mastercard unfortunately.
Source? See what I wrote above. Amazon have blatantly just realised they have enough transactions to bully Visa and are using this as leverage. If you don't think they've already negotiated the lowest transaction fees of all retailers on planet earth then well...