They are willing to serve my general-purpose computer a stream of data over the wire. What I do with that stream of data is none of their business. It's as simple as that. If they want me to view that data only if some conditions are met, it's their right - but they have to enforce it before sending the data, and not retroactively complaining afterwards. There's no moral issue here, except publishers trying to guilt-trip people into viewing ads, because they're too scared of actually asking for money[0]. They want to have their cake and eat it too.
[0] - rightly so, but that's a feature of capitalism, isn't it?
Even getting entirely away from the argument that ad networks can host malicious code, it's quite simple. I send a burst of information to a server. The server sends a burst of information back. I then decode the information to reveal a set of instructions.
Demanding that I execute all these instructions without exception is like a subscription service sending me a newsletter in the mail and demanding that if I pick it up, I've somehow agreed to read the whole thing, beginning to end, out loud. Absurdity.
[0] - rightly so, but that's a feature of capitalism, isn't it?