There's quite a bit of difference between "free" and "no additional cost" -- particularly when
Amazon has already said they
plan on passing some of that cost (the non-Amazon related data traffic parts) along to their customers at some point.
There is no EVDO Fairy that makes EVDO access out of nothing with just a swish and flick of her wand:
somebody pays for it. You can bet that
Sprint is billing Amazon, though it's probably at a ridiculously low bulk rate. If Amazon were to decide that it's costing them enough money, you can bet your sweet bippie that they'll start passing that cost along to their customers with a warm smile. Only Santa Clause can operate without revenue in excess of costs.
That being said, I'm
not especially concerned that Amazon will start charging for book downloads any time soon (general web-surfing is a different kettle of fish). I expect they're viewing the cost of the download in the same way they've been viewing the cost they eat on free "Super Saver Shipping," only less so, because even at a relatively high rate of something like 1¢/MB, a book
download is going to cost them a whole hairy heck of a lot less than mailing a paper book would.