He used peace because the bigots seeing "inferior violent savages" organizing peaceful protests made them more uncomfortable than if they were violent. It's not because he thought violent protest was outright bad, just not as useful in his circumstances. He worked along side of organizers who did use violence. His approach was likely strengthened by this. Also, so many of his protests were called riots by the media. If you take a stand against the monied, they will use it to make your movement appear violent and evil even when it's not.