It's "free" in the sense that it's not a subscription. Not free in the sense that Amazon pays the provider (AT&T in the US) to provide the access, the costs for which are rolled into the business model -> Amazon's fees from the publisher -> Content prices.

So you end up paying for it in the end, since say, $0.05 of every dollar (totally made up figure) ends up going to AT&T.

So, to coin a somewhat wordy phrase, it's free-as-in-jet-fuel-when-you-buy-a-plane-ticket.