This is business. Apple and Google bring something to the table and they meet in the middle. For zero dollars Apple could just let the user choose from a randomised list which is arguable better and more impartial.
The other option is that Google does not pay to get the Apple users, which might come otherwise, but to prevent Apple to try something funny, either directly or indirectly.
In any case, the "build a better product and people will come" mantra is flawed when companies in the space pay each other billions for distribution.