In Europe and the US the main sources of terrorism are i) the far right, ii) Islamic extremists, and then a long way behind iii) animal rights and environmental extremists.
If you haven't seen far-right violence it's because you're not looking. It's pervasive.
The entire motivation for this change at Facebook is because someone just finished killing 50 people in the name of white supremacists and streamed it. You would have to bury your head pretty deep to not have heard anything about this.
Given multiple, recent mass killings (as well as additional disrupted plots) with explicit far-right motivation that have gotten widespread, in some cases global, news coverage, it is more than just something that vaguely exists.
> But most of the videos I have seen online show antifa initiating violence.
That really says a lot more about your selection of information sources than ground truth.
> Are we just more willing to accept their behavior because they are fighting for the "correct" side?
I think we're more willing to accept their behavior because there aren't bodies being piled up in job lots due to antifa-initiated or -motivated violence.