> I think you can answer this question yourself :). This happens all the time in real world contracts. Some unforeseen, unspecified circumstances crop up, and an arbiter or judge decides whether and to what extent they fall under the existing terms. My personal opinion is that if this happened and it was a condition not explicitly spelled out in the terms, it would not be a breach. Because you basically acted in good faith, and factors beyond your control prohibited you from complying.
It's easy to invent more complex issues that render the enforcability of the contract suspect. For instance, I use NoScript. If your ToS notice relies on JS, as most of them do, I won't even see it. Have I agreed to the contract? What if my browser is configured in such a way that I can't see your ad, but I haven't specifically blocked it? Were the precise technical requirements explicitly stated in your ToS? If not, how would I even know how to comply? Even if your legal system recognizes such clauses, they are utterly unenforcable as a matter of fact, if not as a matter of law.
> I think the basic moral principle is this: You create a thing. You say to the world "Hey world, i'll let you consume my thing, if you do this other thing". The world has the right to say "No thanks, we don't want your thing, because the other thing is too onerous". They do not have the right to say "The other thing is too onerous, but we're going to take your thing anyway, thanks."
It's certainly a moral principle, but I doubt you'd find many people who would agree that principle supersedes every other consideration. Even in the crazy world of the U.S. legal system, copyrights and patents have limitations and expirations. If you create a thing and publish it, you certainly have a limited say in what happens to it, but you don't get to dictate terms in perpetuity and at whim, nor should you. In a society that prioritizes the rights of creators to the exclusion of everything else inventions have very little value.
In fact, one could argue that in the case of websites we care so (comparatively) little about the public interest precisely because most websites offer little of significant value.