The law protects peoples' rights in certain ways. However this does not mean that those legal protections limit the definition of those rights.
The rights are in fact more expansive than the laws which protect them. Hence just because something is legal does not mean it is not a violation of rights.
Many cases of 'deplatforming' are in fact a violation of the right to free speech. If a group of megacorps which own the systems we use to enact discourse and make a living decides to do everything to prevent you from using those systems, they are violating your rights.
The law is not the same as public consensus, and it does not have the same guidelines/requirements.
I don't support punching nazis, but that is not the same as deplatforming them