I think there's an implied contract from not your acceptance of the content, which happens before you necessarily know the terms, but from when you can see it and the ads[1].
Regardless of how you feel about specific cases that may be a monopoly, that doesn't work as an argument for running an ad blocker for general viewing, as it's obvious every site you visit is not a monopoly.
This is effectively a matter of social breakdown. I wouldn't bother blocking ads if they were few, privacy-respecting, responsible, etc. But, tragedy of the commons and all, shitty ad-pushers and privacy-invaders ruined the game. Now, sorry to say, this hurts others who try to be more respectful. Not their fault, but that's how the world goes sometimes.
If everyone used an ad-blocker, it would change the way the market works. It would actually reduce overhead where advertisers are fighting for attention in a noisy world. It would restructure how the flow of money works. We'd figure out other and better ways to deal with funding things.
So, I don't think it is wrong to use an ad-blocker. I think it is socially responsible. Even when other people fail to use adblockers, it gives more power to the advertising industry, and thus hurts society. I say: Thank you for doing your part and using an adblocker!
There are cases where the business model of ads has near monopoly. I use Pump.io but most people and connections are on Facebook / Twitter etc. — it's completely unacceptable for those entities to demand that they have power over censoring my access to interact with my friends who those companies have captured into their system. I don't want Facebook or Twitter at all, I want to interact with other regular people in the world. I'd prefer to do it outside those platforms and do when I can. Blocking ads on those sites is perfectly reasonable, a tiny defense against powerful offensive entities — this is not an exchange between two parties with equal power making an agreement.
Meanwhile, efforts like https://snowdrift.coop are in the works to fund creative work from reasonable and respectful people.