The issue isn't that they're deleting accounts as they see fit. Twitter, Youtube, and FB are all using the exact same Google search algorithm to sensor the exact same people/accounts across all platforms. That goes beyond privately owned and is collusion for censorship. Private businesses can't collude to set prices and they can't collude to censor to set ideas.
If they had done this individually, you'd have a point, but because they coordinated, they are liable to antitrust laws.