Many corner shops have notice-boards in their windows where customers can post notices of things for sale, or planned concerts and so on. If one of those customers used the board to co-ordinate arrangements to torch the garden shed of someone they disliked, the shopkeeper would be justified in taking down the notice, and banning that customer from using the service in future.
If it was left in position, the owner of the shed would believe that the shopkeeper was partly to blame for any damage that was caused - and would be right, as the publishing of the plan and the contact details for anyone wishing to join in would be a major factor. I don't think that anyone would be so crass as to try to support the customer if he/she tried to claim their rights had been violated.
What was organised by Parler via Twitter was just as illegal, and involved many more people and more targets.