True, S3 is not a CDN, but for a lot of use cases, serving directly from S3 is fine.
Taking Vine for example, I picked a random vine from twitter (https://vine.co/v/bE3YI365gxd) and a popular vine from twitter (https://vine.co/v/bEFFxdwjK9x). The popular video and thumbnails are served from a CDN, where the new one with no traffic is served directly from S3.
YouTube did the same thing in it's early days. CDNs are not required for all traffic, and blindly recommending them is a bad prescient. They're really great for content that is frequently accessed, but their value greatly decreases on long-tail content.