I have my Discover cashback tied to Amazon, too, only I had to proactively accept the service. There was no behind my back and, besides, the cashback is only applied to the Amazon balance if I so specify.
You seem to have made a mountain out of a molehill.
Maybe I will take credit for their now getting permission first. This was 2 years ago or longer. I was only browsing Amazon, not even placing an order and my screen on an item told me how much less the item was with my cashback applied.
I was livid.