@Skullgrid I don't really know why they do that but my best guess is that joining others to express anger at a third thing is a form of social bonding.
I used to have several neighbours who would watch a weekly television program on animal abuse. As far as I can see, the experience of gathering next to the letterbox and denouncing the animal abusers gave them a sense of community.
It may also fulfil the dual function of reassuring them that the world has plenty of people who also don't support the hated object.