> By visiting the site, you're entering into an agreement
No.
By replying to my browser, they're entering into an agreement to not waste my time, and give me lots of money. Oh, not that either? Hmm. Several sites detect that I'm using a malware blocker. I make no effort to obfuscate this. Some websites will ask me to turn it off, others will block me if I do not. That's fine - I've no obligation to them, they have no obligation to me. Neither of us entered into any kind of agreement.
Keep in mind - I'm even happy to unblock and click ads relevant to my interests and follow relevant affiliate links - to stuff I'm actually interested in. Webcomics in particular are my jam.
So I would likely not even bother with ad blockers if site owners were not routinely acting as a proxy for criminals in (attempting to) serve me malware, extortion-ware, pyramid schemes, and other scams - or peddling horrifically low quality untargeted advertizing that doesn't respect my time (think "same 30 second unskippable ad plays 5 times in a row" utter and other absolute garbage.)
> Legally I have no idea. But ethically/morally, I'd argue you don't
If you want to talk ethics and morals, just how ethical and moral is it to leave your computer open to joining a botnet - imposing so many externalities onto other parties not privy to your "agreement" in the form of DDoS attacks and proxying to obfuscate the origins of such criminals? I think these concerns far weightier than the ethical or moral concerns of depriving a website owner a few cents for selling my time, my attention, my browsing history, etc. (even if they've outsourced that messy work to an ad company who does so on their behalf.)