Reading through the terms and conditions, the following makes me somewhat nervous...

14. Fees
As consideration for you becoming a member of the Rewards Program, you shall pay to Rewards Network any applicable fees (including membership fees, if applicable) charged by Rewards Network from time to time, along with all applicable taxes thereon. We will not charge your Registered Card for any fees without providing you advance notice.

I just get a feeling in my gut that at some point an email is going to go out about a membership fee and anyone who fails to respond will find a little present on their CC bill