This seems unnecessarily constraining. They already allow you change your preferences to see 100 results per search instead of the default. How does this fit with your logic?
Certainly Google is very concerned with click-through on ads, but I'm not sure they'd be losing much if they allowed continuous scrolling. What percentage of their revenue do you think they derive from ads placed on 'next' pages?
And certainly there would be some way that they could integrate more ads with the Ajax loads: adding them to the sidebar as you scroll, or otherwise integrating them with the incrementally added results.