I have a few things. First, I'm not certain I would agree with your characterization that, for example, telephone companies are liable for the content that travels through telephone lines. A core principle of carriage is that a provider of a service is not liable for the content provided through the service, much like Canada Post is not liable for carrying a letter that contains libellous or otherwise problematic content.
This is a principle that developed and informed what we call the “safe harbours”, particularly in the United States, with respect to online platforms, the concern being that if online platforms are liable for the content of their users, they will not actually be able to function. We deal with such a volume of content. Google indexes trillions of websites every single year. Then there's the sheer volume of content that Facebook deals with. We would not possibly be able to manage that. If we were liable for that content, we wouldn't be able to function.
I take your point in terms of Mr. Greenspon pointing out that he'd be liable for a letter to the editor, but he as editor-in-chief chooses to put that letter up. If we have to pre-approve content online, that means the Internet basically comes to a screeching halt, because you just can't deal with the sheer volume of it.