Where does this idea come from that if someone is abusive then they must know what they're talking about?

Personaly when I see an abusive post I get the impression that they don't know what they're talking about and are simply repeating something that they beleive in but don't have the evidence to fully back up. Someone who know's what there talking about is capable of expressing there view without abuse because their confident enough in their knowledge, and know that nothing in life is black & white anyway.

Regards, Anthony