Piers Fawkes of PSFK explains how his site plans to adapt if newspaper websites decide to charge for articles:

Our readers value the aggregation of old-media stories we do on their behalf and so we will simply pay for subscriptions to those titles and continue to publish excerpts and comments for free. That should mean a boom in our traffic as readers avoid paying to subscribe to various publications.

Pay walls limit the dissemination of articles and, as a result, the conversation that examines, analyzes, questions, and ultimately enriches those articles. Scott Rosenberg (co-founder of Salon):

When you put up a pay wall around a website you are asking people to pay more for access to material that you are simultaneously devaluing by cordoning it off from the rest of the Web. So others can’t link directly to it, and the article is unlikely to serve as the starting point for a wider conversation beyond the now-narrowed pool of subscribers.

By excerpting from those cordoned-off articles, PSFK can, under fair-use, enrich its own articles and make them into the starting point for the wider conversation to which Rosenberg refers. That will make PSFK even more valuable to its readers, and potentially draw in many new readers, both of which make the site more attractive to its advertisers. As for the websites that charge for articles, they stand to lose much of their audience to sites like PSFK, and a smaller number of paying readers can’t make up for that:

If the Times and/or Post were to erect a pay wall, I see things playing out as follows: they lose most of their readers; ad revenue declines accordingly; the revenue they make from readers who do pay won’t even make up for the lost ad revenue; and so by switching from free to paid access they’d actually sink further into the red.