The "cash back" is only partially literal. They don't give you paper cash, they deposit the amount into your "Apple Cash" account (look in your "Wallet" app for this account), and then you could just use it from there or you can transfer it to a checking account. It also doesn't happen instantly, it's usually a few days after the transaction officially posts on your card account.
They don't just make it an X% discount because they're trying to encourage you to actually use their card. I'm not sure they can legally differentiate at time of sale the discount based on what card your using, but giving rewards afterwards for using your Apple Card and/or Apple Pay.
I'm not sure where you're located but in the USA this "cash back" concept is very common in consumer credit cards. All of my cards have at least a 1% on ALL purchases, some purchases in categories get more. For example, I get 5% on everything on Amazon with my Amazon Chase card. My Discover card rotates categories of 5% cash back quarterly; last quarter was on everything from PayPal and all restaurants. This quarter (Oct - Dec) it's everything from Amazon, Target.com, and Walmart.com (not sure if in-store counts for those....).