People like recommending things. Linking — whether it’s done on a web site, via email, or word-of-mouth — is a fundamental activity. It’s an activity that gets rewarded. The Drudge Report does nothing but recommend news with their links.
When you hide your information behind a pay-wall or registration-wall, you’re penalizing people’s money or time for access to your stuff.
Charging for your content penalizes all involved in the linking / recommending of that content.
Popularity: unranked [?]
That might be the point, though. I can’t help see, reading through all the imagined scenarios of “How can we get readers to pay,” this misunderstanding of how a story actually moves through social circles (online and off) and how many different news sources a person may browse through with barely a thought to branding. Paywalls, in every call for them I can remember, assume the homepage is much more of a destination than it really is. Many seem to see only one route to a story.