Censorship is a valuable tool I agree, there is epistemic responsibility to try and control output of dangerous idea. However, is very demanding to try and read minds beforehand and stop dangerous idea getting out all the time.
If man is sharing dangerous idea because true then he should be willing to face consequences of speaking truth to power and being a shot messenger. Take vaccines or climate change problem. Do you think it economically realistic to preempt and censor every denier? Censorship is used often but why not for these things when so much evidence shows antivacs are hurting themselves and others as are climate change deniers?
Is because of who is doing censoring.
Rules and laws might not be arbitrary but sometimes people who create them are not right people. When not right people, censorship is enemy of public. In both where is and isn't being implemented.
Internet has made mad men more crazy in that it gives them giant megaphones. When only a few people can hear and see the crazy you don't have to censor. When people are removed from the crazy and read or hear of sane sounding middleman then the idea change from harmless to dangerous.