There is no such notion - private companies have to obey the laws of the land like anyone else.
Platforms like Twitter have the right to ban politicians on the basis of the rights of private property and freedom of speech and association. The same rights that allow restaurants to eject people for "no shoes, no shirt, no service" and allow radio stations and newspapers to choose what and what not to publish, and me to tell Jehovah's Witnesses off. I don't know why this suddenly makes people feel uncomfortable, when these rights, and the ability of private enterprise to exercise them, have been part of the basis of Western liberal democracies for hundreds of years.
The inverse of this would be to give carte-blanche ownership and rights over all property to politicians - including social media platforms, that supersede the rights and desires of the platform owners. That it would be illegal to ban any politician from any private property under any circumstances.
I believe it's a good thing that the President of the United States has no more right to act the fool on Twitter than you or I should. Twitter is not, and should not be, the sole nexus for all global political and cultural communication. It's a microblogging platform, ffs, the only reason it "matters" at all is because one specific paranoiac President didn't trust his own media apparatus.
It's a convenience. It's certainly useful, but it isn't necessary.