(full disclosure - I posted this on reddit about 6 years ago, just saw it while I was deleting my reddit posts and thought I would port it over)
So I'm at this grocery store that I've just started going to. They have one of those free rewards card programs which I'd just signed up for the day before.
Checkout cashier is a grumpy woman. She doesn't make eye contact with the customer she's checking out, and she's constantly talking to another coworker. As it gets to my turn, she tells her coworker: "last one!"
So after scanning my items, she asks me for my rewards card (which would give me about $8-12 off for that purchase). I don't have it, so I give her my phone number (which I'd seen other people do). She tells me my phone number isn't in the system.
Me: "Oh, but I definitely signed up for the rewards card before."
Her ("C" for cashier): "When?"
Me: "Uh... 2 or 3 days ago?"
C: "It's not in the system yet."
Me: "Oh. In that case... can I still get the rewards discounts?"
C: "No, I need your rewards card for that."
At this point, the lady behind me offers to let me use her rewards card, so I take it, thank her, and give the card to the cashier.
C: "That's not yours."
Me (getting annoyed): "I know. I have my own, but it's not in the system and I don't have it now. It's a free rewards card. What's wrong with using her card?"
C: "You can't do that."
This was the point where all the cashiers were changing shifts, so her coworkers were leaving and new ones were taking over.
C gives a long, audible sigh and says: "My shift is over. If you don't have a rewards card you have to pay the full price. That'll be $X."
I suddenly have a stroke of brilliance as I remember how painfully slow the registration process was for my rewards card.
Me (smiling now): "You know what? I lied. I don't have a rewards card. But I'd like to sign up for one!"
This supermarket had a weird system for keying in the rewards card number - customers filled out a form on the spot and the cashiers filled in everything on their terminals. It took forever.
I got a form and filled it up. I gave a fake name, email, and phone number. I made my name and email as long as the maximum number of characters, and even gave an optional (fake) address.
I stood there smiling as the cashier (who looked like she was about to blow) typed in everything.
By the time I was done paying, she looked like she was going to murder me regardless of the fact that we were in a large supermarket.
As she hands me my new rewards card, i tell her as I walk away: "Nah, you keep it. All the information I gave you was fake anyway."
I had to rush back to my apartment with my groceries to make my next appointment after that, but man i felt so good for the rest of the day.
Ah, yes, they don't allow it because it muddles their data. Never forget that whatever rewards they give you aren't free, you sell your data to them for that reward. They want to know which age group or gender buys which products how often, so card sharing messes with their data because suddenly all the "young lady" purchases are on the card of an old man or whatever. It's actually not a huge problem if it happens once because they anyways need hundreds if not thousands of data points to learn anything from it (because they look at "average" purchases), but the cleaner the data set the easier it gets.
There were already trials where shops could predict if you are pregnant based on your purchases. How Target Figured Out A Teen Girl Was Pregnant Before Her Father Did This card data is pure gold (if enough people participate), which is why stores love starting their own program to collect or join a big card provider who will share that data with them.
Oh i know. Even better for the store if they have an app, even more data to collect.