Sit-ins are an old protest tactic. They were used during the civil rights movement, in private property. They were used during labour protest, in private property. They have repeatedly been successful at achieving the protestors ends. They have repeatedly, in hindsight, been viewed in a favorable light and broadly seen as permissible. They were frequently, at the time, viewed as illegitimate and impermissible. Of course, there were other sit-ins which failed and were very much unpopular and were unjustified.