Burried deep in Terms and Conditions it says "22. You agree to pay us any fees (including membership fees) that we charge, from time to time, in connection with the Program (“Fees”), along with all applicable taxes thereon. We may pay fees or provide compensation to Third Party Providers and others with respect to your participation in a Program."

I called them
RewardsNetwork: we don't charge fees.
Me: Yes, but you can decide to at any point.
RewardsNetwork: Yes, but we would email you first
Me: where is that in the terms and conditions
RewardsNetwork: It's not, but trust me, we would.

They force you to register a credit card, so it seems to me that this could turn into a nasty scam at any point. I doubt they would do it as long as they are affiliated with the American Airlines Miles Program, but if that relationship dissolves? Not sure I am signing up.