Coming from someone who believes that the AWS egress fees are highway robbery, I'd throw this out there. In order to maintain settlement free peering, you need to keep egress and ingress somewhat even with your peers.
Amazon proper probably has 3x egress vs ingress between prime and twitch. AWS is an easy avenue to balance it out.
All that being said: I would guess there's almost 0 chance that's what's happening here. Amazon is just using it as a form of lock-in IMO.
I agree that it is just lock-in. The price of egress is the same even when using devices like the Snowball where the data is loaded onto a server and physically shipped to you.
Amazon proper probably has 3x egress vs ingress between prime and twitch. AWS is an easy avenue to balance it out.
All that being said: I would guess there's almost 0 chance that's what's happening here. Amazon is just using it as a form of lock-in IMO.