I'd imagine that Google has acquired a right, rather than a duty, to serve advertising through their partnership agreement and in trade has given partners a rig

I'd imagine that Google has acquired a right, rather than a duty, to serve advertising through their partnership agreement and in trade has given partners a right to some of the money from that. Certainly I wouldn't impose a duty upon myself unless I absolutely had to - and Google is likely to have had the majority of the leverage in any individual negotiation. In any case though, I imagine that the agreement would be far more acceptable given an only slightly modified version of business as usual as the alternative, than it would given an ultimatum between this and death.