It's all about implicit vs explicit contracts. Blocking ads on your own device is a breach of an "implicit contract" that the website owner thinks you have with them, but that you've never entered into or even seen, because it's not written out anywhere, it's "assumed". Riding public transit without a ticket is breaching an explicit contract with the transit agency that you entered into when you physically entered their property, and that is usually printed out and posted somewhere. In some places it's a municipal law as well.