I don't see how that follows. Why would it? That x% of users represents X amount of users who click ads, which means y% of lost revenue. Why would the monetary value of that y% be so different to another search engine vendor?

Either it's scaling effects (owning a few percent makes you more known, which drives new users) or Yahoo is better at selling ads through those users. Note that in the former case, there's even more incentive for Google to block. And if the second is true, oh boy, they certainly have a problem at G...