Apple subjects it's user to a highest bidder. You can't seriously try to claim Apple is using Google as "best search engine" minute after seeing they are getting 15B for it.
Google pays Apple to set it as the default as its a positive sum to both sides. Google gets more than $15bn of value from the traffic and Apple gets $15bn for the obvious decision. Google could call Apple's bluff and refuse to pay, but Apple could pull through on their threat to replace default search with an inferior engine. They did it with Google Maps by replacing it with Apple Maps, which was a widely unpopular decision
