While I agree with you he has a history of smearing conservatives, in this case he smeared no one. He merely distanced himself from an ad, that while admittedly an effective and fair ad, also carries certain risks of blowback. By distancing himself from it, he reduces those risks, and yet the ad still runs. Not only that, but by the act of distancing himself from it, he gives the ad more exposure.
McCain's move is a win-win.
Stop being so naive.