[cratermoon]

The carrot to get you to sign up for the loyalty card is the promise of discount prices. The shelf price says $X, but "Members pay just $Y". Of course $Y is closer to the true price. Sometimes they'll send out a mailer with extra coupons, usually somewhat customized based on your past purchases. But now they get to track all your activity. This is not nefarious to the extent it's used to plan inventory and purchasing, but to the extent that your store profile is sold to other companies, it becomes nefarious.

A casual reading of the terms and conditions might lead a person to object, that it says they don't sell your data. A close reading notes that it says they don't sell to "third parties". But they leave out the fact that any other company the store does business with is not a third party. They technically don't sell your data to them, they provide your data to other companies during the normal course of transacting business with them. Thus, your data flows through the system, unchecked.