alt Hacker NewsI think you're missing a small yet significant portion of people who will pay for content after consuming it. Off the top of my head I've "tipped" after the fact for: An awesome free work time recording app, "free" walking tours when I'm traveling, free online chess, Wikipedia — all after using them.
Most people won't do that though. The walking tour is an example of a situation where there is likely a societal expectation to tip. No such thing on the internet where the publisher is a faceless unknown.