The reason it's difficult to differentiate is because currently media backlash is actually the only accountability any of these tech companies have for doing the right thing. For example, there's basically no government regulation for making sure companies encrypt your passwords properly; the only incentive to do so is because the media would make a joke out of the company if unencrypted passwords were ever leaked.
So you can ban advertisements from political organizations, but if a third-party or the Russians start running misinformation ads on climate change and pro-life, the media won't exactly recognize it as a "non political advertisement" and leave off.
In the election of 2016, most of the misinformation was not directly initiated by the political candidates. Your suggestion wouldn't affect any of the misinformation spread in 2016.