I’m not familiar with this so take this with a grain of salt because I’m just guessing. Let’s say your CC is with Visa and T-Mobile has this offer going. You sign up for it and the fine print says that Visa gets to send T-Mobile details of your purchases. How much information I don’t know. They might just send a total of eligible purchases or they might send a lot more. Then, Visa sends them some amount of $ for each eligible transaction which is more than the cash back they’re giving you. Visa is ok with this because the more purchases you make the more interest you’re likely to pay them so everyone is making money but you.
Edit: in other words they’re counting on you paying more in interest than you’re getting in cash back.