It varies. For example, for www.amazon.com, I get www.amazon.com -> tp.47cf2c8c9-frontier.amazon.com. -> d3ag4hukkh62yn.cloudfront.net.
Why is that?
(My first thought was, wow, surprised they outsource that.)
I'd wager at least part of it is the same reason many companies use CloudFront: Separate point of failure in case of downtime at the AWS side, some one else shouldering DDoS.
Pure speculation, but probably the same reason they ran Oracle for so long -- not many cdn providers when amazon.com launched. Afaik akamai still has one of the biggest edge networks of all the providers.