I don’t see this as being wrong in the context of the DNC. It’s not a government suppressing free speech. The DNC had selected Hillary Clinton to represent them, and they had already moved on to the next objective, beating Trump. Bernie was realistically out of the race at this point. The total vote count and the delegate count just didn’t add up in his favor. Having people yelling and screaming to disrupt the convention (even if they have good points) weakens the DNCs ability to beat Trump. From the DNCs point of view I can understand them not accepting that disruption.
If we were having a meeting with neighbors about a proposed gas pipeline running through the local park, and one the neighbors started jumping up and down and screaming about too much street runoff entering the creek in the park, I’d want to hear what she/he had to say, but at some point I’d want to move on to talking about the pipeline. I see the same kind of thing happening at the DNC.