Looked to me like they simply opened the gate. From what I could tell, Bonnie and Clyde's house happened to be the first one they encountered. Not exactly sure about the legal properly-line definitions, but they were on the edge of the sidewalk bordering their lawn. Because (I think) they were the first house, Bonnie & Clyde didn't have a point of reference as to what these people were doing or their intentions (as houses down the street would have).
Had they been, say, the 14th house, and had the couple seen the demonstrators pass the other 13 without incident, this probably doesn't happen.