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Apart from the law I don't know how or why such an obligation on the part of the media would arise. The media is not a vocation or a calling, it is a business. I don't see any overriding moral imperative imposed on the media, such as the Hippocratic oath imposes on doctors.
Journalism is not a vocation? Medicine is not a business? How romantic.
Look, Glenn did not call for any legislation to compel Comcast to accept political ads. And as far as I know, only candidate's ads must be accepted. But as libertarian as I am, I would be willing to investigate the wisdom of compelling media conglomerates that hold an effective monopoly to air bona fide political ads. (Perhaps with accompanying legislation providing that they may not be sued for any political/issues ads; that only the sponsor is subject to suit.)
Where a cable company does not hold a monopoly, there should be no state interference. But some municipalities are still giving essentially monopoly status to Comcast, for a variety of reasons that technology will soon render moot, but hasn't yet. Where that is true, it is corrupt for the primary media outlet to be able to keep political speech from the electorate.