Efficiency for starters. Efficiency. How many people does it take to efficiently complete a task? The guy looks to resist somewhat initially, so while the police eventually went overboard, the guy could have made things much easier and likely avoided the violent confrontation. It's pretty hard to know exactly what happened and why it played out that way without being there in the moment though.
I think the police are great in general. I'd much rather have them around then not. I also have a few close cousins who are cops and while at times they can smothered with praise, others they are treated as the criminals. They are treated worse more often then they are treated better by the public, yes, but that's something known going into the job, and nobody is forced to become a cop.
The overall point is if you have this many police dealing with a situation like this, no wonder why they are short elsewhere. It doesn't take 5 or 6 electricians to screw in a light bulb, and if 1 guy is installing, while 5 are standing there watching, that's a considerable amount of more useful work not being completed elsewhere.
So if you have incidents happen all over during a protest, how many cops do you need to settle it down. In a crowd of thousands how much is to much or to little. Who determines that once things jump off and how long does it take to know the situation. Everything is happening in real time and communication is complex during these events. Either way, I see your point but I am not sure if we are arguing the same issue.