You are going back to the time of NetZero, who was probably one of the first companies to offer free Internet, at first anyway. They had an ad that was on top of everything and you couldn't avoid it. Though... many of us had that tool to hide it, but that's not a topic for this discussion.
"Ask HN: electrical tape or blue painter's tape, which is better and why?"
They're still around and still offering free dial-up!
Let's say you want to give this free internet to marginalized communities - remote pacific islands, for example. Maybe you could provide internet to the islanders for free, and get paid by multinational companies to do it, for the PR value. As far as the world is concerned, it'll be Tostitos or Jiffy Lube providing internet to Easter Island or wherever... but on the ground it'll actually be you.
You can sell transfer. It's the content provider that pays for the bandwidth, so it doesn't matter if it's free for the end user, as long as you can keep the marginal cost low enough for scale.