I tend to agree with you. He looks undercover. And that's not a casual hold or grab.
From the political rallies I've gone to, I can tell ya that besides all the uniform cops & snipers, there are a number of undercover cops in the audience. I wouldn't have noticed them--but then I realized that two of the guys just trying to blend to the crowd in were neighborhood cops from my old neighborhood. One was carrying a backpack with some serious metal inside.
He's not. The hold in the picture happens to be "banned". That particular grab and hold happens to be deadly force. It is intended to knock someone out quickly, or kill them. The thug's presentation was not at all that of a trained cop either. He's a union thug.