Dan Gillmor gives plenty of reasons why newspapers shouldn't charge for archives. Here's another, plain and simple: Open newspaper archives are good for the community.
Forget monetization, forget maintaining newspapers' authority, forget being higher than competitors in search rankings. Journalism exists, in its golden ideal, to spread truth and give people information that helps their lives. Journalists should advance that cause as far as possible.
At the newspaper I work for, for instance, we keep our Web archives open in full, dating back to late 1989. Links don't change, we offer a free archive search and we link to old stories frequently, to provide context.
I bring this up because a few weeks ago I saw a cool example of how our free archives had an effect on people. There's a legally blind homeless man in town, and nobody knew how he became homeless -- until an old friend searched for his name on the Web and found our site's coverage of him.
Pretty cool. And I'm sure this sort of thing happens all the time, thanks to open archives.
Helping the community -- making the world a better place -- is why a lot of people went into journalism, believe it or not. Keeping a newspaper archive open increases the chance that journalists' work has an effect on people's lives.