It's not up to them to impose severe consequences on the spot, outside of life-threatening/terrorism scenarios, because a colleague has been assaulted. The level of force used should be appropriate to the threat faced.
You do not American History X stomp on an assailant lying face down on the ground after they have just been tasered. He also had no interest in actually restraining the man after doing so, as he moved on, and you can see the one he stomped on moving freely again later in the clip, so he became a threat again. This could imply negligence and a lack of adherence to proper procedures.
It's easy to think 'I'll never get into trouble so I don't care if this level of force is used'. However, we've seen from other countries where that leads once you allow your police force to increase the use of force and get away with it, often on people who've done little and sometimes even nothing wrong. Maintaining strict standards prevents a slippery slope into more widespread and accepted brutality.