I made money by using a discount card as a cashier
Is this something that has just been implemented?
Not really. Many people use them nowadays.
If that is something becoming more common, I do not see a problem with it. It's not like you were breaking the rules or anything.
I guess its common since more people don't mind about it
It sounds like the rules are becoming more relaxed about it. I see no problem as long as it is allowed and legal where you work.