Sites could easily refuse to serve content to people who run adblockers or who turn off Javascript.
Instead, by serving their content upon request, the sites are implicitly agreeing to my terms. So, I'm not doing anything wrong by running an ad-blocker on my computer.
I think it would be great if someone codified this too. If servers can have Terms of Service, so can users. Wouldn't it be great if my browser could send a TOS to each site once before I accepted content from them? A simple notification of my terms, via a custom header sent from a browser extension would work today, but I don't feel that I need to do this since most servers happily give me their content.