But, the revenue Apple get from the deal/trust with Google is better than MSFT or Yahoo ever got by competing with them. It's also easy, risk free and pure profit. So, the deal essentially ensures that Apple will not compete.
Apple now also have an interest in maintaining their (alleged) trust. It's now in both parties interest to maintain Google's dominance of the space. It's also in both their interest to have a large ad market. Competition is not necessarily head-2-head. If Apple couldn't make money by funneling user to Google, they might prefer to just suppress ads. I'm sure if they asked users, users would want that. "Phone with fewer ads, ad-related tracking, etc.) is a potential selling point.
IANAL, and more to the point, I dislike both legal framework around antitrust and the intellectual framework implied by much of it. There's just a lot more to monopoly than either take into account.
Such deals are, IMO, evidence that monopoly^ in general is to the detriment of competition, innovation, churn and other such positive market outcomes. I also think more monopoly is concerning on "free culture" grounds, it perculates to politics/government, etc. Lets leave those aside though.
Apple & Google themselves were only able to innovate and prosper because monopolies did not yet control the microcomputer or the web. App stores, OTOH...
*Meant broadly. Read, Trust, cartel oligopoly, monopsony, bigness...