Eventually I saw a pattern, surprising since it was common in major cities all across the US and as if there were some single, central training materials. Apparently:
(1) Police are taught to be in control of any contact with a citizen. Recently the police have been taught to act nice initially, but, once it is clear some actual law enforcement is to be involved, be in control.
Being in control can mean that the citizen has been intimidated and made submissive so that they won't resist. Part of this is to demand that a citizen DO some little things, e.g., stand with feet apart, move back 10 feet, or tolerate being falsely accused of something, e.g., "weaving" in the road, being too close to the officer, etc. The officers are looking for things, even trivial, fake things, to object to so that they can object. It's like Captain Sobel in the 101st Airborne training in the series Band of Brothers -- "find some" infractions so that can complain about them and force the soldiers to accept being falsely accused so that they will be more compliant -- the police seem to have borrowed this tactic.
If the citizen does not look submissive, then the officer provokes a defensive reaction from the citizen so that they can arrest the citizen or threaten to arrest them.
Then, finally, maybe arrested, the citizen has been subdued and is submissive, which is what the police wanted to begin with.
(2) The police like to teach citizens, to change their attitude, and do this by hurting them, e.g., hitting them with a club, bending their arms, throwing them to the ground and putting a knee on their neck, spraying them with pepper spray, etc. They regard good police work as meting out "cruel and unusual punishment", with pain and maybe serious injury, without "due process". So, the police want to be absolute dictators on the streets.
(3) In a confrontation with a citizen, the police want some result where they successfully took some law enforcement action, a ticket or an arrest. E.g., in Atlanta, at first they didn't want merely to leave the citizen alone or, if the citizen was drunk, let him call a cab and (ii) later wanted to make sure the citizen was not able just to run away. The reaction to a citizen running away?
"Shoot them and kill them. Gee, they might 'get away'; can't permit that; that would violate due respect for the police; so, shoot the citizen." -- or some such.
(4) The expected, usual approach to an arrest is to throw the citizen to the ground, hold them down with a knee to their neck, their arms behind their back, and put on handcuffs.
From the 100 or so videos I watched, it appeared that (1)-(4) are so standard that they have been taught from some standard source. E.g., in all of that, some semi-bright guy had the idea that it was good to put a knee on a neck, and it appears that that is now standard.
Apparently part of (1)-(4) is the associated support for it from the Blue Line, e.g., police unions, Police Benevolent Associations, liability insurance cities buy for their police, the norm of police sticking together, local prosecutors, DAs, and judges who work daily with police and want to cooperate, politicians who want safe streets, etc. And at times maybe there has been more to police power, e.g., confiscating cash, shakedowns, payoffs, etc.
I'm sure that changing (1)-(4) can be done but won't be easy.
Police are expected to acquire control of any situation they are called to through seizing initiative. This means that they don't wait for anyone to take any action, but immediately take verbally or physically dominating actions.
It is nearly impossible to acquire control of US civilians without violating the Constitution or Bill of Rights. Once someone has violated the law, the police are protected in terms of this violation, but because they are trained to acquire immediate control, they most often are violating the rights of people who've broken no laws.