Now this video is a bit difficult to explain. It appears that police took an arrested protester, made him kneel in front of them, and then took a group picture in Pittsburgh.
From my own protest cases, I can confirm that police departments will often take a picture with arresting officers for purposes of creating a record. This is usually a picture with the officer who signs the arrest form. I have never seen a group photo op with a perp. Moreover, you usually do not force them on your knees in front of them.