Laypeople often confuse public domain for "free", and vice versa. Most content available for free on the web is copyrighted, which restricts what people can do with it. Often, the content is made available using an advertising model, trading the opportunity to read and interact with content for the user's attention to ads or links to e-commerce websites. But website users are usually not free to republish content or email the content to friends beyond the bounds of fair use. They're bound by whatever terms and condition the website chooses to employ; if there are no explicit terms and conditions, they still can't copy the website's content for other uses.
Even professional publishers are sometimes confused by copyright on the web. In 2010, the editor of "Cooks Source", a Massachusetts magazine got into hot water for republishing a blogger's work without permission. The publisher's response to the blogger, on being asked for restitution, made the rounds of the Internet, and is striking for the bellicose ignorance it betrays:
Yes Monica, I have been doing this for 3 decades, having been an editor at The Voice, Housitonic Home and Connecticut Woman Magazine. I do know about copyright laws. It was "my bad" indeed, and, as the magazine is put together in long sessions, tired eyes and minds somethings forget to do these things. But honestly Monica, the web is considered "public domain" and you should be happy we just didn't "lift" your whole article and put someone else's name on it! It happens a lot, clearly more than you are aware of, especially on college campuses, and the workplace. If you took offence and are unhappy, I am sorry, but you as a professional should know that the article we used written by you was in very bad need of editing, and is much better now than was originally. Now it will work well for your portfolio. For that reason, I have a bit of a difficult time with your requests for monetary gain, albeit for such a fine (and very wealthy!) institution. We put some time into rewrites, you should compensate me! I never charge young writers for advice or rewriting poorly written pieces, and have many who write for me... ALWAYS for free!
Many free e-books are available on a similar basis as free websites. They may include advertising or advocacy. Promotional literature and instruction manuals often fall into this category. Many publishers make free e-books available for limited periods of time as a means of marketing them; that doesn't make them free to redistribute, though it happens.