I should note that I generally don't post entire articles from newspapers or magazines, because that would probably infringe the copyright in the articles. But I don't have such a hesitation about letters to the editor, the copyright in which is generally owned by the letter's author, not by the newspaper.
With letters to the editor, I think that there's probably an implied license to redistribute the letter further, since broad distribution (for free) is what the authors of such letters usually want. And even if there's no implied license, the republication is fair use, because the republication doesn't interfere with the letter-writer's market for his work: Letters to the editor almost invariably make no money for the letter-writer (whereas newspaper articles do make money for the newspaper, through advertising income).
This is naturally an oversimplification. When I teach copyright law, we spend days on fair use, plus some time on implied licenses, and I didn't want to go into all this detail here. But I wanted to note this for those who wonder why I'm willing to copy entire letters but generally not entire articles.