Google may be afraid that Apple would develop their own search engine, just like they developed their own maps engine and iCloud for email/photos. Apple, with their massive piles of cash, would have the resources to do that.
Or buy one, like Bing, and take it from middling to solid.
yes, the same way they went with Siri.
Or that Apple simply changes the default Bing, offering something less but greater than 0, and Google loses out on >20 billion in ad revenue.