In America, some stores seem to share rewards member information to product companies so they can target their ads on other platforms. For example, I paid for a very specific candy from CVS using my credit card, but used my partner's CVS rewards member number and they got ads on facebook for that candy while I haven't seen a thing.
ANY bonus card is basically paying you to be tracked.
It’s the core mechanic.
Even here in privacy conscious Germany the largest bonus card, payback Punkte, is simply giving you a 1% back just to get you to log your own purchase.
I'm not disagreeing with you. I'm actually inclined to agree. But this hand-wavy "this should be illegal!" type of comment is extremely common on these threads and is completely unhelpful.
All uninformed data sales feel icky to me, but data sales that involve actual payment history even more so. There's incredible amounts of personal information int he stuff you buy for your day to day life, ranging from religious items to medicine, and coupled with an account from a party like Google or Facebook, this pretty much completely deanonimises the otherwise anonimisable internet profile.
You can get a new internet provider and a new Gmail account, but your purchasing habits won't change and are easily mined. A real-life connection between an online account and the stuff you buy in physical stores crosses a boundary that I think should not be crossed
