The billing redundancy argument would be applicable generally but in the case Netflix on AWS is not something Netflix has to worry about.
Netflix is a marquee AWS customer. The PR damage of Netflix even making noise about leaving AWS would be terrible for the service as it fights for market share against Azure and Google. Netflix will get their way.
Exactly, if Amazon wants to keep AWS as the backbone of large parts of the internet, they need to be able to prove other business considerations don't affect it - if they ever treated Netflix any differently than another marque client they'd torpedo the trust in them from any organisation who think they might compete in the future.
Netflix is a marquee AWS customer. The PR damage of Netflix even making noise about leaving AWS would be terrible for the service as it fights for market share against Azure and Google. Netflix will get their way.