Is adblock stealing? Do you use adblock? Why? Sure, advertisers don't have the right to your attention, but don't the content creators have the right to your financial support?
I never agreed to terms with random websites which stipulates that I must view their ads to view their content. So no, it's not stealing. Websites can detect ad block in most cases and will block content if they feel strongly about it. Ultimately, the widespread use of ad block should indicate that customers don't like it, and it would be in everyone's best interest to find another way.