How does a supermarket knowing my personal shopping habits negatively impact me, in concrete terms? I can only think of only one hypothetical downside: the supermarket sells my shopping data to malicious third parties, which adversely affects me in other ways (e.g. my health insurance premiums rise because I bought too much bacon). This would indeed be very bad, but it is currently a mere hypothetical.
As far as I can tell, supermarkets offer discounts in exchange for more fine-grained tracking of customers’ purchasing habits because it lets them better optimize their supply chains, nothing more, nothing less. I fail to see how this is a bad thing for customers.