1) People will pay for e-mail--if it becomes an additional service, as opposed to "included" (as it currently is).
In other words, when you sign up with AOL right now, you get up to five screen names, with an e-mail box for each name. That set-up for the e-mail boxes is an auto-script for the mail server, as opposed to the basic web server, that can easily be disabled.
So the terms of agreement with an ISP would change. You'd get the basic service--access to the web--for the normal fee, but to get e-mail, you'd agree to pay extra.
2) Many spammers don't run their own web service with direct access to the internet--too easy to get your net address blocked that way. Most of them sign up with different services, do their batch processing, then look for the next service to sign up with and do batch processing from there. As soon as an ISP realizes it's being used to propagate spam, it usually determines the user doing it and terminates their service--unless the spammer is paying for the bandwidth bite, and the ISP is okay with it.
So it's not as simple as setting up your own web server.
3) Johnv points it out: Private enterprise.
Look at the history: Juno and NetZero both used to offer unlimited free internet access, in exchange for us having to put up with annoying advertisement banners. It was simple: You connect, then leave their ad banner stuff in the background while you open a new browser window and do your thing for as long as you want.
NEITHER offer free internet access anymore, although NetZero is still calling themselves "NetZero". In fact, although I'm not looking real hard these days, I can't find a single company that offers free internet access. It's just not profitable.
Bill Gates knew exactly what he was doing. The original Microsoft vision statement was: "A computer on every desk and in every home, running Microsoft Windows." And that's almost true these days. And who gets the profits from this shoddy piece of programming? Bill Gates. Everything the man has done has been from a profit margin point of view.
So watch this latest one, boys and girls. The billionaire swami of computerland has had a vision of the future of profiteering, and made the suggestion that we should pay for e-mail. And when E.F. Hutton talks, people listen (some of you older folks will know the reference).
I give it another year at the most.