There’s the tough guy cop thing. There’s also simply the fact that cops are human, and some people are curious. You’re a cop who normally does traffic stops or deals with crazies on the subway. A call goes out asking if anyone wants to join the perp walk. How many are going to say yes, just for the sake of being part of something that clearly is a historic event? Who wouldn’t want to see the CEO killer with their own eyes? Or maybe, you go just to say you were there. There’s always some number who just want to be able to say they had a part in history, however minor. I bet there were a bunch of sheriffs and other law enforcement who told the tale to their grandchildren of that time they met John Brown.