For starters, your company is not the largest video provider in the world, like Netflix. As someone else mentioned here, diversification can be for "political" reasons as well. Especially if you're as large as Netflix; diversifying could keep your billing in check. Let's say you split between AWS, Azure and GCS. That gives you the ability to scale based on what is cheapest. You could also do public and private clouds in tandum. You can scale your private cloud/on-premise to the public cloud if need be with most large cloud providers (though I never suggested they use on-premise.)
As far as this not being Amazon's primary business, well, they are a lot bigger and diverse. But let's use a Wal-Mart analogy. Wal-Mart is so powerful (maybe not anymore, because of Amazon) that if it doesn't like your wholesale pricing to them, they can move your shelf space and practically destroy your business. They have the leverage in those relationships. Now, with AWS being the #1 cloud provider you have a similar lock-in, at the very least your switching cost would be quite high.
So, let's just for example sake say Amazon make a decision to give their own video streaming priority on their own cloud. That's not breaking net neutrality laws, because...it's their servers, this has nothing to do with telecom. So now Amazon Prime Video, streams 4K at a much better rate than Netflix (and Netflix would likely never know.) Then, perhaps another assumption, AWS decides to increase their pricing tier for their media servers for streaming. So suddenly they have squeezed you in two ways. Quality of Service and pricing.
Amazon could easily do a calculation to compare themselves to Azure and GCS to tell what is the proper amount they could get away with and still make it more expensive just to migrate to a competitor. You're locked-in, and just to make up for your switching, it would cost, let's say 1 years worth of AWS service. Hard to explain your sudden blip in earnings to short minded investors.
Anyhow, all I am trying to say is...it would make more sense, to me, from a business stand point to diversify across multiple providers instead of being all in on AWS. I never said you had to have an on-premise set-up. It would be much different if they were a small start-up, but Netflix is not. It can at times take up more internet traffic than torrenting.
I will point out though that services like Docker are making cloud lock-ins harder to do, but they don't solve reliance on APIs or just proprietary offerings. There's a reason why Google, Microsoft and Amazon all underline, push and constantly improve those offerings as a form of lock-in.
It is possible Netflix has some sort of pricing agreement with Amazon that locks in a rate for x amount of years. But either way, when you're as large as Netflix, I think diversification is the better long term strategy.