The supply-side has defectors too.

I go on a blocking-spree for any site which (A) serve malware (B) unexpectedly plays sounds or epileptic animations (C) trashes site performance through a billion third-party loading items.

Even if it's just a lack of vetting by the ad-network, N-levels removed from the author, it represents a breach in that same completely-unspoken contract which I will take steps to protect myself from.

Finally, let's not confuse game-theory with "justice" or with our normative beliefs about how things should work.