They don’t have to allow their platforms to be used to incite violence or insurrection. That’s their choice. You’re welcome to go to the cesspool that is Parler if you want to make violent threats.
This is a major flaw in modern legal tradition and social morality. Society will continue to suffer from it. People need to stop pretending Internet media is an optional luxury. It's now integral to the public's extended mind and method of social/political discourse. No one is writing letters to the editor anymore, or going to salons, or likely talking heavy politics with their neighbors.
If an elected President of the country is acting deranged then that's the responsibility of congress to do something about and not of glorified CRUD app creators.