How points rewards cards are directly marketing us without us knowing

Have you ever signed up for a points reward card? Many of the big retailers such as Tescos, Superdrug and Boots offer this and some such as the Nectar card, allows you to collect and spend points on the same card with multiple retailers. But have you ever studied the vouchers or emails that you have been sent in a little detail. It is not purely coincidence that you buy a particular brand of dog food then the following month receive a 20 pence off voucher for it.
These reward schemes are tracking what we purchased or spent our points on so they can directly market us based on this information. They can even track where we are based on shops and petrol stations that we have used our points card at.
Some people refuse to use points cards as they feel this is an invasion of our privacy, but this does mean that they are missing out on saving those all important pennies by opting out.