How, if at all, you complain depends on how your content is being recycled.
As we've covered, sometimes a backlink to your site makes a complaint detrimental, and you may instead leave a friendly comment to thank the site owner for their attention. This is a chance to groom a contact while clarifying to readers the source of the material.
Sometimes you may like the idea of having an extract of your content being showcased at the site, but would prefer the webmaster to clarify or enhance your attribution. A friendly, professionally toned e-mail to the site's webmaster should suffice.
Sometimes there's no benefit, and you want content removed. Here are some approaches.