They can't. The games provide them with another source of income. Basically, you provide your name, address, etc. when you set up the card. Then every time you use the card, your purchases are recorded. Then drug stores, supermarkets, etc. collect this information under your own name and sell it to interested parties, like insurance companies. Your insurer uses that information to raise your rates.
It's all here:
Here is a site against the use of such cards and data mining: