Essentially websites such as YouTube remain free because, in exchange for allowing you to watch videos, you spend a small amount of your time watching adverts.
If you use AdBlocker then essentially you are using the service without paying for it (paying with your time). You are stealing.
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Maybe if ads weren't so intrusive, people wouldn't block them. Problem is not with consumers, the problem is the advertisers. Also, you should sit in on the local middle school and learn the definition of stealing. Obviously you have missed that lesson.