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.