In the background of this poster is a piece of a letter written by Martin Luther King from his jail cell after he was arrested in Birmingham, Alabama in 1963. In it, he defends his belief in nonviolent protest as well as one's moral responsibility to break unjust laws.

Here's some more background...
"The Birmingham Campaign began on April 3, 1963, with coordinated marches and sit-ins against racism and racial segregation in Birmingham, Alabama. Circuit Judge W. A. Jenkins issued a blanket injunction against "parading, demonstrating, boycotting, trespassing and picketing". Leaders of the campaign announced they would disobey the ruling. On April 12, King was roughly arrested with Ralph Abernathy, Fred Shuttlesworth and other marchers—while thousands of African Americans dressed for Good Friday looked on. King met with unusually harsh conditions in the Birmingham jail. An ally smuggled in a newspaper from April 12, which contained "A Call for Unity": a statement made by eight white Alabama clergymen against King and his methods. The letter provoked King and he began to write a response on the newspaper itself. King writes in Why We Can't Wait: “Begun on the margins of the newspaper in which the statement appeared while I was in jail, the letter was continued on scraps of writing paper supplied by a friendly black trusty, and concluded on a pad my attorneys were eventually permitted to leave me. The "Call to Unity" clergymen agreed that social injustices existed but argued that the battle against racial segregation should be fought solely in the courts, not in the streets. They criticized Martin Luther King, calling him an “outsider” who causes trouble in the streets of Birmingham. To this, King referred to his belief that all communities and states were interrelated. He wrote, “Injustice anywhere is a threat to justice everywhere. We are caught in an inescapable network of mutuality, tied in a single garment of destiny. Whatever affects one directly, affects all indirectly… Anyone who lives inside the United States can never be considered an outsider…” The clergymen also disapproved of the immense tension created by the demonstration. To this, King affirmed that he and his fellow demonstrators were using nonviolent direct action in order to cause tension that would force the wider community to face the issue head on. They hoped to create tension: a nonviolent tension that is needed for growth. King responded that without nonviolent forceful direct actions, true civil rights could never be achieved. Against the clergymen’s assertion that the demonstration was against the law, he argued that not only was civil disobedience justified in the face of unjust laws, but that "one has a moral responsibility to disobey unjust laws."

This is not to say that I feel street art comes anywhere close to being as vital a rebellion as the civil rights movement, rather just as an illustration of something bigger. That there will always be resistance to re-shaping the world in any way. If you want to make a difference, you can only do so by pushing through those who will try to dissuade you, be it with indifference, mockery or aggression. In this day and age it is important to look back at history and see what kind of difference a positive, non-violent- yet unwavering quest for change can create. 

There are people out there who aren't interested in anyone altering the status quo. This is true in cultural issues as well as individual ones. If you want to change the world at large- or just your own world, life or circumstance, don't wait for someone to give you permission.