United didn't, but it was at their request. I've read conflicting reports of the rights of the airline/passenger, but most of it deals with denying boarding at the gate, not forcing a seated customer off the plane (unless they were disruptive, which doesnt apply in this case).
I can't know for sure, but I'd bet my best buck that United didn't request a passenger be beaten. They asked that he be removed and the officer took it from there. I do agree this was handled inappropriately from a PR standpoint, but that doesn't mean the can't legally remove you from their plane.
That argument will never fly. He was an agent of united, no matter how you frame it. And it isn't clear that united could forcibly remove a seated passenger who wasn't posing any safety risk.
Based on what I've read, he was an agent of the airport, not United. Regardless, it will come back to United though, because that is how it's being portrayed. Airlines reserve the right to remove any passenger for any reason they see fit, while it may not be logical, they can do it.