Censorship is not just harmful because those with power are deciding what ideas others are not allowed to publish. That is censorship's lesser evil. The Great Evil of censorship is that those in power decide what YOU are allowed to hear.
Often when the word censorship comes up those in the de-platforming camp are quick to cry that Twitter, Simon Schuster, Google, Facebook, Amazon, etc. are private companies. To their minds, this means the heavy handed role for these powerful central authorities in shaping public opinion lately is not to be questioned.
Well, that's bullshit.
Censorship is censorship whether done by governments or private institutions or any other organization that is powerful enough. It doesn't even need to be all that organized. When activists in the classroom spontaneously shout down a professor so other students can't hear what he has to say that's censorship.
And here's the thing. It's at least as harmful for dissent to be silenced by private entities like big tech. Likely it's more harmful because private companies tend to react to powerful forces unhinged from democratic will, such as their bottom line. Or the political inclinations of top executives.
Now, to be clear, I acknowledge that Twitter and Amazon have a legal right to censor.
But that doesn't make it the right thing to do.
As 
We are living in a world where everything from payment providers, to domain name registrars, to hosting providers are being weaponized against people attempting to understand the world by sharing information. All of this is done “in the name of protecting people”. Apparently people are so weak-minded that they are easily led astray and manipulated by “false” information. This so called “false” information is a danger to society! People might revolt against the status quo and demand something “bad for us all”.
That's an excellent point. This alone should be enough to dissuade censorship and de-platforming. But there's a more alarming angle.
Censorship ends dialog. Without dialog political means to compromise are taken off the table. In the end censorship, public or private, will only fan the dissent it aims to smother.