"I'm Joe Schmoe and I approve this message"
Ok, but about when Joe Schmoe doesn't approve the ad? What about when it's a fiercely negative ad with a disclaimer at the end that says something like, "This as is produced by 'People Against the Other Guy' and is not reviewed or approved by any candidate"?
Do you believe that?
I do not. Call my cynical, but I believe that even the most negative ads are pretty much always reviewed and/or approved by the candidate that is opposite the one being slammed. The one they want to look good in comparison. I know the history behind this point in political campaign evolution, but IMO nothing has changed. It's still candidates running negative ads, but now with plausible deniability to cover their arses.
And really, unless the candidate comes out and specifically disavows the ad, AND seeks to correct it, they've given complicit approval, anyway.