I think it's a question of who your users are. When people build on Facebook or Twitter, their users are the same users as the platform's. Zynga and Facebook both are pulling from the same pool -- it's pure competition. When you use AWS, you are very directly Amazon's customer. They have a much stronger incentive to support you, because you're a direct revenue stream to them.
Plus Amazon is a large business with many divisions. If you compete with Amazon Instant Video, the AWS folks aren't going to like having their profit centre sacrificed for the sake of the video division.
