From the few examples being shown on the internet, there seems to be a great reluctance to touch a subject being arrested and/or a great reluctance to getting uniforms messed up in the process.
I was a police officer in the 70's. We did not have fancy toys or bullet proof vests. What we did have were slap saps, short billy clubs and kubatons (a keychain billy-club). We learned there was a difference between inflicting pain and incapacitating a person when using these tools.
With a lot of practice we became quite efficient at taking all kinds of people into custody with minimal effort. If you were not good at learning the skill set you got your ass beat. Then you had to work harder to take that person into custody.
You also have police officers that are on power trips. They intentionally say or do things to incite a violent reaction of a citizen contact. This type of officer thinks they have the right to do this just so they can administer street justice and prove they are in control.
I am also happy to say these officers are in a very small minority.