Amazon probably isn't paying 15 cents/megabyte in the US. But they're probably paying significantly more outside the US. They've figured out the average cost per megabyte and probably added on a few cents. This way the author is getting charged something in the neighborhood of $2.70 to deliver each book rather than being charged $0.50 for some and $15 for others depending on where the purchaser happens to be. Also keep in mind that Amazon lets you download purchases again at a later time via 3G, so they're probably factoring variable delivery costs based on the number of times people are normally downloading the books.
Amazon also has a pretty good track record at lowering costs when their costs go down (at least when looking at AWS), so it wouldn't surprise me to see the delivery fees dropping over time as they negotiate better deals with AT&T or if they start working directly with other mobile operators in other countries.
Amazon also has a pretty good track record at lowering costs when their costs go down (at least when looking at AWS), so it wouldn't surprise me to see the delivery fees dropping over time as they negotiate better deals with AT&T or if they start working directly with other mobile operators in other countries.