That logic is deeply flawed. If content is made available for free, regardless of whether or not it's ad supported, it's still free. I'm not stealing anything if the website owner is making the content available to me at no cost. If I were to employ this logic in every day practice, I'd have to read every single ad so as to ensure that I'm not "stealing" anything from anyone. Now, if I was stealing an internet connection, I'd agree but since I'm not actually taking something that requires payment, I'm not stealing anything.

If ad blocking mechanisms truly were "stealing," there's no way that Google, a company that makes most of its money through advertising, would allow ad blockers to be distributed through the Chrome Web Store (more here). In fact, many of those extensions are targeted at Google products.