I wish Bernie showed some backbone, kinda disappointed he just let them take over his event like that.
What was he supposed to do? Shove them off the stage? He stood there respectfully for 20 minutes, the arguably best thing to do.
Calling security would have been the best coarse of action
That's the problem. These people are protesting about police brutality so he can't do that. Even if it's a private security firm, it's still going to look very bad. These people are obviously not going to go down quietly, they will resist being taken away, they may even hurt themselves in the process. Say they do end up getting a twisted arm or a Black eye in the process of flailing around, in the current atmosphere of the left, even if it's apparent that they did it to themselves, you can't say anything negative about Black activists. The whole story will be "BLM activists injured by security at Bernie Sanders rally". He won't be able to refute it, he knows that, that's why he never criticizes these people, he just walks away and says nothing, that's all he can do.