Expose the logic. Put a "?" Icon button on the add that lists every data point used in the decision making process, who collected the data, when, and how.
E.g:
1.1.2020 @ Starbucks on main st in 90210 you bought a medium coffee, a gift card, and a cookie.
1.3.2020 @ Barnes and noble you bought a book and m&MS.
So when the customer wonders why they're being shown advertisements for cat litter they'll be able to see how the ml algorithm thought they'd be interested.
If the logic can't be explained in real time to the advertised too then it surely can't be justified to the actual companies buying said advertisement.