It's a pretty common, unappreciated practice, if there's a copyright notice they probably are strongly against, people to routinely quote news reports from major sites, standard etiquette is to cite the source of the text. Site (lots of blogs) that encourage you to reprint their content provide RSS feeds that can be setup to automatically show their latest work on your site.
Article directories like Ezinearticles.com act as libraries of ready made content for sites, in exchange for your getting free content you are expected to retain the author credit including a link to their site.
A practical point, search engines tend to not give duplicate content as high a search ranking as the originator, the first one they find may be assumed to be the original author.
Software "Spinners" can automatically scramble an article to make it appear an original work to search spiders, this can help it earn higher search ranking, but can be damaging in the event of an increasingly common human review.
There are services like copyscape.com that check for duplicates of a text, don't expect your 'borrowed' content to go unnoticed.