Criminal justice, sentencing, and police behavior  --  especially behavior toward African Americans  -- have become contentious issues.  As one would expect, they have shown up in the race for each party's Presidential nomination.  They have opened a window on (1) what treatment is given speakers with opposing viewpoints, and (2) what, in some circles, is considered an "opposing viewpoint."

Democratic presidential candidate Martin O'Malley apologized on Saturday for saying "All lives matter" while discussing police violence against African-Americans with liberal demonstrators.

Several dozen demonstrators interrupted the former Maryland governor while he was speaking here at the Netroots Nation conference, a gathering of liberal activists, demanding that he address criminal justice and police brutality. When they shouted, "Black lives matter!" a rallying cry of protests that broke out after several black Americans were killed at the hands of police in recent months, O'Malley responded: "Black lives matter. White lives matter. All lives matter."

The demonstrators, who were mostly black, responded by booing him and shouting him down.