[This is not legal advice.]
Copyright is a protection granted to the author of a work “fixed in any tangible medium of expression”. Meaning, something that can be recorded somehow. It extends to writing, music, painting, architecture, and many other forms of expression.
The web has transformed the ease by which we can share and copy information, and violating copyrights has become a simple act, taken for granted by many. Just ask the Recording Industry Association of America.
Copyright grants exclusive rights to reproduce your “works of authorship” to you, the author. You get these rights automatically upon creating the work, although there are some extra benefits to registering the copyright. You might have signed these rights over to an employer, sold them to a publisher, or waived some of them by releasing your work under an alternate copyright license. Otherwise, they generally last the lifetime of the author plus an additional 70 years.
Specifically, writing on the web has been dancing around issues of copyright for ages. Sharing information from another site, even with attribution, could often be construed as a violation of copyright. Increasingly, services like Readability and Instapaper are taking site content out of it’s original context and making it available elsewhere. This can prevent a site owner from generating ad revenue obtained by visits to their site.
Often, these services are seen as acceptable to authors where they can increase the reach of their work, and drive more traffic to their website. The acceptance of the author doesn’t mean these uses of their content aren’t in violation of copyright protection, but if the copyright holder doesn’t enforce the protection, nobody else will.
I’m not aware of any of these services being taken to court over infringement. This leaves little in the way of legal precedent in regards to the boundaries of acceptable practices when copying written content online. It falls to writers to pursue legal measures when they feel their rights have been violated, but most online writers lack the means to prosecute a claim.
One day a line will get crossed that will make online writers take action, but until that takes place, the extent of infringement will only continue to grow.