It's all about the money
They're having difficulties suing the actual originator of the objectionable content so they're going after the common carriers.
It's different for news outlets who actually write the articles. They have some responsibilities to review what's being printed (or posted online). Advertisements are a halfway house. The news outlets are able to filter for some things such images or specific words but don't have the capability to review the claims made in each ad. That applies to to products, including that of politicians if the claim is made within an ad. If it's made within an article, the journalist and the news outlet must be careful to quote the source and if they feel safer to add a caveat.
Once money becomes involved, scope creep comes in. Why not sue the postal service for delivering junk mail rather than go after the actual originator? The current pressure on Google, Facebook, Twitter, etc. is purely because they are seen as easier targets with deep pockets than going after individuals.