They can ban individual accounts calling for violence (which happens on both the extreme left and extreme right). But it's only socially acceptable to ban one of those, the other gets a free ride.
Would you agree that it is Twitter and Facebook's responsibility to ban all BLM-affiliated groups? After all they have organized numerous events that involved mass rioting that resulted in billions of Dollars in damage, many deaths, and disruption to the lives of hundreds of thousands. In many cases, politicians like AOC or news pundits like Don Lemon have condoned this violence, and even encouraged more such action. At times they have gaslighted us by claiming those events were "mostly peaceful" even though there is evidence to the contrary showing criminality far larger in scale than the recent capitol riot (example: https://www.theguardian.com/us-news/2020/may/31/fires-light-...). Those individuals and groups who organized, incited, and participated in such events were not reprimanded and not banned.
So why are they all given a pass? It's because these tech companies are comprised mostly of left-leaning staff and are willing to now exercise their power against everyone else. Until Twitter, Facebook, and Amazon deplatform every single person and group involved in the hundreds of events in 2020 that involved criminal acts, these new acts of censorship will remain discriminatory and unacceptable.
And I frankly don't care much for your example. My grandfather was a Pole who had to fight Nazi invaders, and I have no reservations about him having to put a few bullets over them.
Nazis and fascists do not want a free and fair society. They delight in the violence of it and have no interest in a marketplace of ideas.
The radical left is a problem in Seattle as they are terrorizing the Capitol Hill neighborhood.
Don't take my word for it. Listen to community activist Victoria Beach explain how she is upset for them terrorizing the area and hijacking the Black Lives Matter movement.