It might be a thinner line than you think: At the end of the day, governments and private companies are both just big entities with strong influence, specially in monopoly situations. Consider this: If ebay and amazon simultaneously ban one specific book, author, genre, etc.; how much would that decrease their reach?
There's two different aspects to be considered here: What you seem to be mostly worried about is direct legal consequences to speech, like being imprisoned for saying the wrong thing. Protecting people from that is important, but freedom of opinion and expression is more than just that.
But more important than that is the societal aspect: censorship is ultimately a propaganda tool. You don't have to imprison people to silence ideas; simply hiding them from the public is more than enough.
The reason this is the more important of the two is because it is what influences public opinion. Propaganda is an incredibly powerful political tool and freedom of expression is, also an opposing force to it.
Admittedly this is going to be a really shitty analogy, but in some cases your reach is so decreased that it's effectively gone.
If you want to host some incredibly terrible text or blog, and magically no DDoS protection provider at all wants to protect you because you're such a terrible person, your reach is 0%, you are knocked off instantly and down 24/7. The government or state never had to say a thing.
In many parts of US, Walmart might be the only store people have access to, it is infeasible to get anywhere else. Having to drive several hours to the nearest city is practically reducing reach to nothing.
This obviously doesn't apply to people that can pop on Craigslist and pick up a copy of a book, or order it from eBay, or any of the hundreds of other shops that might have it. But there is a huge amount of the country where you are quite literally down to one or two retailers. Partially due to those one or two retailers completely taking over the area and outpricing every small shop there is.
