They believe that their actions are justified because they believe that they are morally superior. They are self-absolving. That is the hallmark of extremists of all types.
That’s why.
Why do you think that about them? Is that based on anything that’s actually in reality?
It is obvious that they feel justified in inconveniencing or endangering hundreds or thousands of people at a time by blocking roads and bridges, invading the stage at performances, etc. Any one of those people could be rushing home to meet their children, or to a hospital to see a dying parent, or facing the loss of their job if they’re late. An inattentive HGV driver could plow into the back of stopped traffic and kill people. But, it’s totally ok because their motives are pure. That’s self-absolution. That is the hallmark of an extremist.