That thing is more expensive than an xbox 360. I think they've got enough profit margin to cover the wireless. But that's what I think. I'm not going to post a "fact" without having any proof to back it up.
It also depends on the profit sharing they have setup with Sprint. My best guess is that Sprint gets a slice of the device revenue to cover 'setup' of the device, each time you have a document sent via wireless to the Kindle, Sprint probably pockets all 10 cents of that, and book sales pocket Sprint another 10 cents or so each.
You don't have to pay entirely up front if you bake the cost of delivery into the content as well.