As someone who is frustrated by this, and who years ago also joined now-deleted FB groups where ISIS advocates shared propaganda, yes I am equally frustrated. I believe the value of preserving and maintaining open discussions on the internet outweighs the silencing of them for political aims. In the case of Parler, they do actually moderate some content, and I believe it is conceivable they would adopt a policy to ban violence, but it's too little to late now.
Judging by the downvotes, it would seem the parent comment's request was not asked in good faith. That's disappointing, because I really meant what I said. Obviously extremist groups are problematic, but it doesn't serve us well to be ignorant of their views. One huge example of this is the Trump supporters who had no idea that extremist elements were planning an armed insurrection - by censoring that, all they have to go on is the word of Twitter, which is an organization they already distrust. I have not seen it with my own eyes, but if you look at the screenshots of "hang Mike Pence" discussion happening on Parler, there are less than a dozen people involved in the conversation. That is a very small number that doesn't do much to show that extremists on the platform are a threat justifying the censoring of the service. Again, all they have to go on is the analysis of organizations who they already distrust, who have said Trump's simple statement that he will not be at the inauguration is somehow incitement. Most Trump supporters do not buy that logic, but they might if whatever extremist conversations occurring openly were used as proof to back it up.