I'm almost certain they could offer it cheaper and have chosen not to. If you look at AWS' history they tend to price higher than they need to and then aggressively lower the prices after the service is established. I've always assumed this is part of a very conservative business plan where they establish the actual cost of running the service (as opposed to projections) over time. Once they've established they can actually run the service as cheaply as projected they lower the price accordingly.

Remember Amazon isn't a startup. It is an established company with shareholders to keep happy. Given that I think the above strategy is a sensible one (and is probably why AWS has been profitable from the very beginning)