The linked article said "Target assigns every customer a Guest ID number, tied to their credit card, name, or email address that becomes a bucket that stores a history of everything theyve bought and any demographic information Target has collected from them or bought from other sources."
You might have gotten customized coupons but just didn't notice it! The article also said that the customized coupons are now designed not to freak out customers who think the company may be spying on them: Then we started mixing in all these ads for things we knew pregnant women would never buy, so the baby ads looked random. Wed put an ad for a lawn mower next to diapers. Wed put a coupon for wineglasses next to infant clothes. That way, it looked like all the products were chosen by chance."
I suppose you are right though.... if you take the right steps, you can probably avoid getting mined for data from stores. But unfortunately, it's getting more and more difficult to do so it seems.