What they are not understanding aout is when protesters attempt to FORCE people to listen, something you have no right to compel in the first place. Thats what people do with theri sit ins or traffic stoppages or building occupations, they are physically forcing people to do something they may otherwise chose not to do, and they phisicality of it alone constitutes violence.
The whole point of a protest is to force people to listen to you. Without that you might as well not protest at all. This is because the whole reason a group of people protest is because that group has tried to give people a chance to voluntarily listen and they feel they were ignored. Therefore, they will become louder and more visible until their concerns are addressed.
Also, the literal definition of violence really doesn't apply here. As we have seen with language many times over, literal definitions really don't mean anything to anyone; it's all about a word's perceived meaning. Given that, you would be hard-pressed to find anyone, including your vaunted police, that would agree with you that sit-ins are a form of violent protest.