CBS is given the use of the airwaves in order to serve the public good, through broadcast television. Increasingly, CBS ( and it's parent Viacom ) seem to be attempting to control the political discussion in the country. Blocking this ad is a perfect example of their hyprocrisy and conflict of interest.
The ad campaign started on the web, and unfortunately looks as if it will end there. This has serious implications for the future role of large corporate controlled media. It seems the web and the internet is proving more and more to be the only place where opposing political views can be openly debated without corporate imposed censorship.