Supermarkets do that. It's actually scary how much data they collect and what they can do with it. Target managed to figure out when people were pregnant: http://www.forbes.com/sites/kashmirhill/2012/02/16/how-targe...

Much like RMS, I use a friend's discount card (and pay in cash) at the grocery store for this reason. Will a purchase of soda and cookies be linked to me and impact my health insurance cost down the road? Will targeted Mountain Dew ads make their way into every website I visit? Probably not, but I'd rather be sure.

The self-checkout machines only ask for the phone number, with no id challenge. I get the discounts + no tracking, my friend gets more gas points, and the store is left wondering why he buys so much toilet paper.

I wonder how long it will be before they start cracking down on this somehow, and how far they'll take it.

And yet people laugh at me when I tell them how I buy as much as possible with cash.

What about when they don't give that info away but keep it for themselves? Still creepy?