Originally Posted by radonc1

Surprise!!
They are following their rules. It says that you don't get cash back for certain purchases, including gift cards.
What they see is someone abusing their card for cash back that they (Discover) shouldn't;t pay and you don't deserve.
So, congratulations on getting 3 years equivalent of cash back before they closed you down.
But don't be shocked that they did so.
As I said, you are not their customer of choice.