It must, but I assume once you get to a certain level of usage then an account manager / AWS gets you a slightly better than the publicly published deal. Is that the case?
Also, I wonder if EC2 is up for a price tweak soon too?
If you have a direct-billed account, and are spending more than 1,000/month, you'll get an automatic price break. At my current $DAYJOB, we're billing around $15-20K/month (depending on traffic) in Amazon Web Services charges, and I believe we're getting somewhere between a 10 and 25 percent price break.
My guess is that they are only using AWS for serving the site and handling the uploads, resizing etc. and they are using another provider for the storage of the images (or they are just directly hosting everything with Edgecast and not using them in Pull mode).
Like people have mentioned above, you can get a bunch of dedicated servers with heaps of storage for a fraction of the price of S3. You loose the flexibility and instant scaling, but the fact is these servers would be used more of a backup and would only be hit by Edgecast to upload them to the CDN.
A reduction in the cost of goods sold does incentivise businesses to apply price discrimination without a negative impact on margins - a $5/month service would target a big, untouched, monetisable market.