What's the alternative? Should Safari choose a default search engine by random per user or per session? Google is undoubtedly the best search engine in nearly all cases. Should Apple subject its users to a weaker engine just because Google has a different privacy policy? You can also change the default search engine.
Why wouldn't Apple accept a payment for bringing billions of dollars in traffic to Google?
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
Weaker... According to whom? I particularly use DDG on my personal devices and only occasionally go to Google. Whenever I'm using a machine that use Google as the default, I often go back to DDG to get results unspoiled by the bias pushed by Google's agenda, SEO and paid ads.
But answering your question, it could let the user choose their search engine as part of the onboarding experience while providing a random preselected default.
For my usage it gives good results, and it has the added bonus of providing links to google et al when you scroll down the results (it assumes that if the first page of results isn't good enough then the user will go elsewhere, so why not provide links to the other search engines?).
Why wouldn't Apple accept a payment for bringing billions of dollars in traffic to Google?