Otherwise I could leave a piece of paper in a coffee shop saying. "Apple stock will go up 5% in the next month - By reading these words you agree not to trade Apple stock in the next month unless you pay me a royalty.". Reading something does not mean agreeing to its terms and if the content does not require agreeing to its terms to access the content, then the terms are may as well not exist until you do.
It doesn't mean you can ignore everything there. If you reproduce the content then you are still liable under copyright laws. If you DDOS the site, you are still liable under tortious interference laws.
But outside of something covered by other laws, there is no legal recourse to just putting "no bots" in your terms of service and then suing any bots you find. You can try to _block_ bots, just like you can hide your sheet of paper when you see a cop getting coffee, but you can't sue them for breaking terms they never agreed to. I don't believe there is actually any case law yet as to whether a bot can even agree to a contract. For evidence you can look at the recent LinkedIn case, which, while it is on appeal, turned on this issue.
But even aside from the explicit contractual terms, even without one, you cannot just run around acting in bad faith, right? If you put up a site called AccuratePrices.com and then for some users you knowingly and intentionally provide false information, isn't that something like fraud?