Okay, so you agree that in the absolute majority of cases users have no legal obligation whatsoever to watch ads, but you're interested in a hypothetical website that has constructive notice for terms of service that specifically include language which says users must watch ads. All right, let's discuss that.
In this case, even though the contract itself is enforcable, it does not necessarily follow that every clause is enforcable. Standard unilateral contract law restrictions apply. To the best of my knowledge, no comparable clause has ever been directly tested in a U.S. court, so we'll have to speculate. However, it is well established that certain types of clauses are invalid for reasons such as being unconscionable (compulsory arbitration clauses are a common example). The extent to which a website's terms of service can restrict your legal right to not download certain network packets or adjust how your browser interprets them is an open legal question, at least in the U.S. (I'm confident no such ToS would be upheld in the ECJ).
Furthermore, if we lived in the (in my opinion, dystopian) world you envision, what happens if due to, for instance, an ISP routing issue the third party advertisements you've "agreed to" aren't loaded onto your computer? Are you in breach of contract? And this is not even touching upon the question of whether a "legal obligation to watch" the content your browser actually displays in the end makes any sense (I don't really see how it could).
Also, while this is an amusing legal question to ponder, I continue to hold that the finer points of contract law in a particular jurisdiction don't really serve to illuminate any underlying moral questions.