You are protecting your long-term livelihood as a content professional. Also, taking online publishers to task for copyright violations may well discourage them from such transgressions in the future. If you nail someone for stealing your content, they probably will not make the mistake of stealing from you again. They also may learn that they can be held accountable for any instance of content theft, and thus may avoid stealing anyone's copyrighted work in the future. Publicizing violations of your copyright can have the same effect.
Word tends to spread about unethical practices in the online publishing world.
A prominent bad example can go a long way toward discouraging similar behavior. In the big picture, enforcing your copyright serves a valuable educational purpose. If a copyright violator didn't know that content theft is illegal and unethical, by enforcing your copyright you are providing a valuable lesson. This can benefit the online publisher, as well as the general community of content professionals. All rights reserved.