I think it was sent to participants in all the mileage dining programs. rewardsnetwork.com is the domain of the program, and the program only works if use a registered credit card, so encouraging customers to register all their credits is entirely normal. You'll note that clicking the "Verify" link just takes you to the regular login page for your dining program (UA in your case and AA on mine) at the proper, secure (https with the right certificate) URL. It's not a phishing scam.