I watched the video. Sky marshals purposefully wear normal clothes, so they blend in with the rest of the passengers on the flight. Having them wear uniforms would kind of defeat the purpose.
The cops are likely waiting until the passenger is off the plane to take him into custody.
I suppose he could have identified himself and displayed a badge before the video started, but nothing about this guy said "police" to me. My experience with airport police says that taking the guy into custody was the last thing the actual police wanted to do. That suspicion is confirmed by the fact that after getting dragged off, the passenger reboarded the plane. Justifiably, real police don't consider themselves airline employees. They look down on both airline security and TSA.
Actually the real police might well have told United to increase the voucher amount like any civilized human organization would do. The gate agent then looked outside and picked out the biggest meanest baggage handler she saw. I really doubt the Sky Marshals want any part of this disaster.
The cops are likely waiting until the passenger is off the plane to take him into custody.